Guiding device and associated system

ABSTRACT

An extreme ultraviolet radiation (EUV) source, including: a vessel having an inner vessel wall and an intermediate focus (IF) region; an EUV collector disposed inside the vessel, the EUV collector including a reflective surface configured to reflect EUV radiation toward the intermediate focus region, the reflective surface configured to directionally face the IF region of the vessel; a showerhead disposed along at least a portion of the inner vessel wall, the showerhead including a plurality of nozzles configured to introduce gas into the vessel; and one or more exhausts configured to remove gas introduced into the vessel, the one or more exhausts being oriented along at least a portion of the inner vessel wall so that the gas is caused to flow away from the EUV collector.

This application is a continuation of U.S. patent application Ser. No.16/469,689, filed Jun. 14, 2019, which is the U.S. national phase entryof PCT patent application no. PCT/EP2018/050278, filed Jan. 5, 2018,which is a continuation-in-part of U.S. patent application Ser. No.15/400,929, filed on Jan. 6, 2017, and claims priority to Europeanpatent application no. 17158280.2, filed on Feb. 28, 2017, and to U.S.provisional application No. 62/596,629, filed on Dec. 8, 2017, each ofthe foregoing applications is incorporated herein in its entirety byreference.

FIELD

The present disclosure relates to a gas flow guiding device and to aradiation source, including such a guiding device. The presentdisclosure relates, for example, to a guiding device and radiationsource for use with a lithographic system. The present disclosure alsorelates to a radiation source including an EUV vessel having an innervessel wall that is protected from debris by inner vessel wall suppliesof gas, and more specifically, methods and apparatuses for providing gasflow within EUV vessels for protecting inner vessel surfaces such as aportion of the inner vessel wall that is gravitationally above an EUVcollector of the EUV vessel.

BACKGROUND

A lithographic apparatus is a machine constructed to apply a desiredpattern onto a substrate. A lithographic apparatus can be used, forexample, in the manufacture of integrated circuits (ICs). A lithographicapparatus may for example project a pattern from a patterning device(e.g. a mask) onto a layer of radiation-sensitive material (resist)provided on a substrate.

The wavelength of radiation used by a lithographic apparatus to projecta pattern onto a substrate determines the minimum size of features whichcan be formed on that substrate. A lithographic apparatus which uses EUVradiation, being electromagnetic radiation having a wavelength withinthe range 4-20 nm, may be used to form smaller features on a substratethan a conventional lithographic apparatus (which may for example useelectromagnetic radiation with a wavelength of 193 nm).

A lithographic system may comprise one or more radiation sources, a beamdelivery system and one or more lithographic apparatuses. The beamdelivery system may be arranged to deliver radiation from one or more ofthe radiation sources to each of the lithographic apparatuses.

Extreme ultraviolet (EUV) radiation is used in applications such asextreme ultraviolet lithography (EUVL). An EUV source may generate EUVradiation by illuminating target material such as tin (Sn) withradiation from a high power laser radiation source. A result ofilluminating target material with laser radiation is the generation oflaser produced plasma (LPP), which may then emit EUV radiation.

SUMMARY

When target material such as tin is illuminated with laser radiation toproduce plasma, a certain portion of the target material becomes debris.For example, target material debris may include Sn vapor, SnH₄ vapor, Snatoms, Sn ions, Sn clusters, Sn microparticles, Sn nanoparticles, and Sndeposits. When Sn debris accumulates on an EUV collector or on one ormore inner vessel walls of the EUV vessel, the EUV collector efficiency,lifetime, and availability may be reduced.

The EUV radiation may be produced using a plasma. The plasma may becreated, for example, by directing a laser beam at a fuel in theradiation source. The resulting plasma may emit the EUV radiation. Aportion of the fuel may become debris, which may accumulate on or morecomponents of the radiation source.

This may result in contamination of the one or more components of theradiation source, which may be difficult to clean. The contamination ofone or more components in the radiation source with debris may lead to adecrease in the performance of the radiation source, e.g. the quality ofthe produced EUV radiation, which in turn can lead to degradation ofperformance of an associated lithographic apparatus. Ultimately, thiscan lead to significant down-time of the lithographic apparatus whilecomponents of the radiation source are cleaned or replaced. It is withinthis context that embodiments of the invention arise.

Embodiments of the present disclosure provide systems and apparatusesrelated to EUV vessels that include inner vessel wall supplies of gas,and more particularly systems and apparatuses for providing flowgeometries of gas flow within the EUV vessel that enable a reduction ofcontamination of debris onto one or more inner vessel walls. Oneembodiment includes introducing gas into the vessel via a showerhead ora curtain flow and exhausting gas from the vessel through an exhaustconfiguration. In one configuration, the vessel is designed to have anasymmetric exhaust configuration. In a further configuration, the vesselis designed to have a symmetric exhaust configuration. It should beappreciated that the present disclosure can be implemented in numerousways, such as a process, an apparatus, a system, a device orinstructions on a computer readable medium, the instructions configuredto implement a method. Several inventive embodiments of the presentdisclosure are described below.

In one embodiment, an EUV source includes a vessel having an innervessel wall and an intermediate focus (IF) region. The embodimentincludes an EUV collector that is disposed inside the vessel andconnected to the inner vessel wall. The EUV collector includes areflective surface that is configured to directionally face the IFregion of the vessel. The embodiment also includes a showerhead that isdisposed along at least a portion of the inner vessel wall. Theshowerhead includes a plurality of nozzles that introduce gas into thevessel. One or more exhausts for removing gas introduced into the vesselare also included in the embodiment, the one or more exhausts beingoriented proximate to the IF region so that the gas introduced into thevessel is caused to flow away from the EUV collector.

In a further embodiment, an EUV source includes a vessel having an innervessel wall and an intermediate focus (IF) region. The embodimentincludes an EUV collector disposed inside the vessel connected to theinner vessel wall, the EUV having a reflective surface that isconfigured to directionally face the IF region of the vessel. Theembodiment includes a first gas source disposed proximate to thereflective surface of the EUV collector having a plurality of inlets forintroducing gas into the vessel. The embodiment also includes ashowerhead disposed along at least a portion of the inner vessel wallhaving a plurality of nozzles for introducing gas into the vessel. Anexhaust disposed along the inner vessel wall at an azimuthallyasymmetric position is also included for exhausting gas from the vessel.In certain embodiments, the asymmetric exhaust may be oriented at adownward leaning angle, for example, toward a direction of gravity. Inthese and other embodiments, the asymmetric exhaust may be oriented suchthat it generally opposes a region proximate to a ceiling area of theinner vessel wall that is gravitationally above the EUV collector.

In a further embodiment, an EUV source includes a vessel having an innervessel wall and an intermediate focus (IF) region. The embodimentincludes an EUV collector disposed inside the vessel connected to theinner vessel wall, the EUV collector having a reflective surface that isconfigured to directionally face the IF region of the vessel. Theembodiment includes a vessel wall gas source disposed laterally at leastpartially along a portion of the inner vessel. According to thisembodiment, the vessel wall gas source includes a plurality of nozzleassemblies. Each of the nozzle assemblies may include a first outlet anda second outlet for introducing gas into the vessel, wherein the firstoutlet is configured to introduce gas in a first direction that is awayfrom a second direction in which the second outlet is configured forintroducing gas. In these embodiments, both outlets are configured tointroduce gas along a perimeter of the inner vessel wall.

According to an aspect, there is provided a radiation source comprisinga chamber (i.e. a vessel) comprising a plasma formation region, aradiation collector arranged in the chamber, the radiation collectorconfigured to collect radiation emitted at the plasma formation regionand to direct the collected radiation to an intermediate focus region, adebris mitigation system configured to direct a first gas flow from theintermediate focus region towards the plasma formation region, and aguiding device arranged in the chamber such that the first gas flow isdirected around the guiding device.

The guiding device may be arranged such that the first gas flow issymmetrically directed around and/or diffused by the guiding device.

The debris mitigation system may be configured to direct a second gasflow from the radiation collector towards the plasma generation region.

The guiding device may be configured to reduce interaction between thefirst gas flow and the second gas flow.

The guiding device may be configured to prevent interaction between thefirst gas flow and the second gas flow.

The guiding device may be configured to prevent formation of a jet ofthe first gas flow towards the radiation collector.

The guiding device may be arranged in the chamber to extend at leastpartially along an optical axis of the radiation collector.

The guiding device may be arranged at or in proximity of theintermediate focus region.

The guiding device may be arranged to taper from a first end of theguiding device towards a second end of the guiding device. The first endof the guiding device may comprise an enlarged portion. The second endof the guiding device may comprise a pointed portion or rounded portion.

The guiding device may be arranged in the chamber such that the firstend of the guiding device is positioned distal from the intermediatefocus region. The guiding device may be arranged in the chamber suchthat the second end of the guiding device is positioned at or proximalto the intermediate focus region.

The guiding device may comprise at least one opening or a plurality ofopenings. The at least one opening, the plurality of openings or eachopening of the plurality of openings may be configured to direct a thirdgas flow towards the radiation collector.

The at least one opening, each opening of the plurality of openings orthe plurality of openings may be arranged on the guiding device suchthat the third gas flow from the at least one opening, each opening ofthe plurality of openings or the plurality of openings interacts withthe first gas flow, for example, to direct or push the first flow of gasinto proximity with at least a portion of the chamber.

The guiding device may comprise a heating element. The heating elementmay be configured to increase a temperature of the guiding device.

The heating element may be configured to increase the temperature of theguiding device to a first temperature at which an increased amount ofthe first gas flow is directed around the guiding device. The heatingelement may be configured to maintain the temperature of the guidingdevice below a second temperature at or above which diffusion of debristhat is present on the guiding device increases.

The guiding device may be configured for cooling by a coolant. Thecoolant may be supplyable or supplied by a coolant source.

The radiation source may comprise a debris receiving surface. The debrisreceiving surface may be arranged in the chamber to reduce or preventdebris from reaching the intermediate focus region.

The debris receiving surface may be arranged to intersect or extendacross the optical axis of the radiation collector.

The guiding device may be arranged between the debris receiving surfaceand the intermediate focus region.

The debris receiving surface may be arranged to extend over or overlapwith at least a portion or all of the guiding device so that debrisgenerated at the plasma formation region is incident on the debrisreceiving surface.

The debris receiving surface may be comprised in, part of or provided bythe guiding device.

According to an aspect, there is provided a method of reducing debrisdeposition in a radiation source, the method comprising directing afirst gas flow from an intermediate focus region of the radiation sourcetowards a plasma generation region of the radiation source, anddirecting the first gas flow around a guiding device arranged in achamber of the radiation source.

According to an aspect, there is provided an extreme ultraviolet (EUV)source, comprising a vessel having an inner vessel wall and anintermediate focus (IF) region, an EUV collector disposed inside thevessel connected to the inner vessel wall, the EUV collector including areflective surface, the reflective surface configured to directionallyface the IF region of the vessel, a showerhead disposed along at least aportion of the inner vessel wall, the showerhead including a pluralityof nozzles for introducing gas into the vessel, the showerhead having atleast one inlet for supplying the gas into the showerhead, and one ormore exhausts for removing gas introduced into the vessel, the one ormore exhausts oriented along at least a portion of the inner vessel wallso that the gas is caused to flow away from the EUV collector.

The EUV source may further comprise a material target region disposedwithin the vessel for generating plasma radiation, the plasma radiationbeing collected by the reflective surface of the EUV collector anddirected toward the IF region for entrance into at least part of alithographic apparatus. Introducing the gas into the vessel via theplurality of nozzles may enable protection of the inner vessel wall fromdeposition of material.

The plurality of nozzles may be oriented along at least a portion of aninner surface of the inner vessel wall in a direction that faces awayfrom the inner surface of the inner vessel wall.

The inner vessel wall may have a conical, cylindrical, or polyhedralshape.

The showerhead may extend perimetrically and laterally along at least aportion of the inner vessel wall.

The EUV source may further comprise an outer vessel wall surrounding thevessel, the outer vessel wall including one or more exhaust vents.

The showerhead may include one or more zones, each of the one or morezones including at least a portion of the plurality of nozzles, each ofthe one or more zones being separately supplied with gas for enablingseparately controllable zones for introducing gas into the vessel.

The inner vessel wall may be defined by smooth surfaces, vane surfaces,or a combination of smooth surfaces and vane surfaces.

According to an aspect, there is provided an extreme ultraviolet (EUV)source, comprising a vessel having an inner vessel wall and anintermediate focus (IF) region, an EUV collector disposed inside thevessel connected to the inner vessel wall, the EUV collector including areflective surface that is configured to directionally face the IFregion of the vessel, a first gas source for introducing gas into thevessel, the first gas source including a first plurality of inlets, thefirst plurality of inlets disposed proximate to the reflective surfaceof the EUV collector, a showerhead disposed along at least a portion ofthe inner vessel wall, the showerhead including a plurality of nozzlesfor introducing gas into the vessel, the showerhead having at least oneinlet for supplying gas into the showerhead, and an exhaust disposedalong the inner vessel wall at an azimuthally asymmetric position forexhausting gas from the vessel.

The exhaust may be further oriented proximate to a first region of theinner vessel wall. The first region of the inner vessel wall maygenerally oppose a second region of the inner vessel wall that islocated gravitationally above the EUV collector. The exhaust may enablegas introduced by the first gas source and the plurality of nozzles toflow away from the second region while the EUV source is operational.

The plurality of nozzles may be distributed at least partially along aregion of the inner vessel wall that is located gravitationally abovethe EUV collector.

The plurality of nozzles may be oriented along an inner surface of theinner vessel wall in a direction that is away from the inner surface ofthe inner vessel wall. The orientation of the plurality of nozzles mayenable a flow of gas that is at least partially directed away from atleast a portion of the inner surface of the inner vessel wall.

The plurality of nozzles may be disposed at least partially along aceiling region of the inner vessel wall that is located gravitationallyabove the EUV collector. The plurality of nozzles may be oriented in adirection that faces away from the ceiling region. Introducing the gasby the plurality of nozzles may provide a diffusion barrier adjacent theceiling region for excluding debris.

The showerhead may include one or more zones. Each of the one or morezones may include at least a portion of the plurality of nozzles. Eachof the one or more zones may be separately supplied with gas forenabling separately controllable zones for introducing gas into thevessel.

The inner vessel wall may have a conical, cylindrical, or polyhedralshape.

According to an aspect, there is provided an extreme ultraviolet (EUV)source, comprising a vessel having an inner vessel wall and anintermediate focus (IF) region, an EUV collector disposed inside thevessel connected to the inner vessel wall, the EUV collector including areflective surface, the reflective surface configured to directionallyface the IF region of the vessel, and a vessel wall gas source disposedlaterally at least partially along the inner vessel wall, the vesselwall gas source including a plurality of nozzle assemblies, each of theplurality of nozzle assemblies having at least a first outlet and asecond outlet for introducing gas into the vessel, the first outletconfigured to introduce gas in a first direction that is away from asecond direction in which the second outlet is configured to introducegas, and an exhaust for exhausting gas introduced into the vessel, theexhaust being proximate to the IF region for enabling gas introduced bythe vessel wall gas source to flow away from the EUV collector.

The first direction and the second direction in which gas may beintroduced by the first outlet and the second outlet, respectively, ofeach the plurality of nozzle assemblies may be oriented at leastpartially along a perimeter of the inner vessel wall for enablingcurtain flows of gas along the perimeter of the inner vessel wall.

At least a portion of the plurality of nozzle assemblies may furtherinclude a third outlet for introducing gas into the vessel. The thirdoutlet may be configured to introduce gas away from the inner vesselwall.

The plurality of nozzle assemblies may be distributed at least partiallyalong a first region of the inner vessel wall that is locatedgravitationally above the EUV collector while the EUV source isoperational. The exhaust may be further oriented proximate to a secondregion of the inner vessel wall that may oppose the first region of theinner vessel wall for enabling gas that is introduced into the vessel toflow away from the first region of the inner vessel wall.

The inner vessel wall may have a conical, cylindrical, or polyhedralshape.

According to an aspect, there is provided a radiation source comprisinga chamber comprising an inner wall and a material target region, aradiation collector arranged in the chamber, the radiation collectorconfigured to collect radiation emitted at the material target regionand to direct the collected radiation to an intermediate focus region, adebris mitigation system configured to direct a first gas flow from theintermediate focus region towards the material target region, the debrismitigation system configured to direct a second gas flow from a portionof the inner wall of the chamber into the chamber, a guiding devicearranged in the chamber such that the first gas flow is directed aroundthe guiding device, and an exhaust for removing gas supplied by thedebris mitigation system from the chamber.

The exhaust may be arranged to extend from a portion of the inner wallof the chamber at an azimuthally asymmetric position.

The debris mitigation system may comprise a showerhead. The showerheadmay be arranged along at least a portion of the inner wall of thechamber. The showerhead may include a plurality of nozzles forintroducing the second gas flow into the chamber.

The guiding device may be configured to reduce interaction between thefirst gas flow and the second gas flow.

The debris mitigation system may be configured to direct a third gasflow from a position at or proximate to the guiding device in thechamber towards the material target region.

The guiding device may be configured to reduce interaction between thefirst gas flow and the third gas flow.

The debris mitigation system may be configured to direct a fourth gasflow from the radiation collector towards the material target region.

The guiding device may be configured to reduce interaction between thefirst gas flow and the fourth gas flow.

The guiding device may be arranged to taper from a first end of theguiding device towards a second end of the guiding device. The first endof the guiding device may comprise an enlarged portion. The second endof the guiding device may comprise a pointed portion or rounded portion.

The guiding device may be arranged in the chamber such that the firstend of the guiding device is positioned distal from the intermediatefocus region and the second end of the guiding device is positioned ator proximal to the intermediate focus region.

The guiding device may be arranged in the chamber to extend at leastpartially along an optical axis of the radiation collector.

The guiding device may comprise at least one opening or a plurality ofopenings. The at least one opening, each opening of the plurality ofopenings or the plurality of openings may be configured to direct afifth gas flow towards the radiation collector.

The at least one opening, each opening of the plurality of openings orthe plurality of openings may be arranged on the guiding device suchthat the fifth gas flow from the at least one opening, each opening ofthe plurality of openings or the plurality of openings interacts withthe first gas flow to direct or push the first flow of gas intoproximity with at least a portion of the inner wall of the chamber.

The guiding device may comprise a heating element. The heating elementmay be configured to increase a temperature of the guiding device.

The heating element may be configured to increase the temperature of theguiding device to a first temperature at which an increased amount ofthe first gas flow is directed around the guiding device. The heatingelement may be configured to maintain the temperature of the guidingdevice below a second temperature at which diffusion of debris that ispresent on the guiding device increases.

The guiding device may be configured for cooling by a coolant. Thecoolant may be supplyable or supplied by a coolant source.

The radiation source may comprise a debris receiving surface. The debrisreceiving surface may be arranged in the chamber to reduce or preventdebris from reaching the intermediate focus region.

The debris receiving surface may be comprised in, part of or provided bythe guiding device.

According to an aspect, there is provided a method of reducing debrisdeposition in a radiation source, the method comprising directing afirst gas flow from an intermediate focus region of the radiation sourcetowards a material target region of the radiation source, directing asecond gas flow from a portion of an inner wall of a chamber of theradiation source into the chamber, directing the first gas flow around aguiding device arranged in the chamber of the radiation source, andremoving gas from the chamber.

According to an aspect, there is provided a radiation system comprisinga laser and (i) a radiation source as described herein or (ii) anextreme ultraviolet (EUV) source as described herein.

According to an aspect, there is provided a lithographic systemcomprising a lithographic apparatus arranged to project a pattern from apatterning device onto a substrate, and a radiation system as describedherein arranged to provide at least some of the radiation to thelithographic apparatus.

According to an aspect, there is provided a radiation source comprisinga chamber comprising an inner wall and a material target region; aradiation collector arranged in the chamber, the radiation collectorconfigured to collect radiation emitted at the material target regionand to direct the radiation beam of the collected radiation to anintermediate focus region; a debris mitigation system comprising a firstgas supply system and a second gas supply system; an exhaust configuredto remove gas supplied by the debris mitigation system from the chamber;wherein the first gas supply system is configured to direct a first gasflow from the intermediate focus region towards the material targetregion or the plasma formation region, the first gas supply systemcomprising one or more openings arranged to direct the first gas flow ina direction substantially opposite to a propagation direction of theradiation beam into the chamber; and wherein the second gas supplysystem comprises one or more openings arranged to direct the second gasflow in a direction substantially perpendicular or tilted under an angleto the propagation direction of the first gas flow.

Other aspects of the method and apparatus for vessel wall protection byone or more gas flow inlets and exhaust asymmetry to improve collectorlifetime in LPP EUV source will be made apparent from the followingdetailed description, taken in conjunction with accompanying drawings,illustrating by way of example principles of the method and apparatus.

Various aspects and features of the invention set out above or below maybe combined with various other aspects and features of the invention aswill be readily apparent to the skilled person.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings, in which:

FIG. 1 depicts a lithographic system comprising a lithographic apparatusand a radiation source according to an embodiment;

FIG. 2 depicts a radiation source including a debris mitigation systemof the lithographic system of FIG. 1 ;

FIG. 3 depicts a simulation of a first gas flow and a second gas flow inthe radiation source of FIG. 2 ;

FIG. 4A depicts a portion of the radiation source of FIG. 2 including aguiding device;

FIG. 4B depicts a further embodiment of the guiding device of FIG. 4A;

FIG. 4C depicts a further embodiment of the guiding device of FIG. 4A;

FIG. 4D depicts a further embodiment of the guiding device of FIG. 4A;

FIG. 5 depicts a portion of the radiation source of FIG. 2 including afurther embodiment of the guiding device;

FIG. 6 depicts the portion of the radiation source of FIG. 4A includinga debris receiving surface;

FIG. 7A depicts a simulation of a first gas flow in a portion of theradiation source of FIG. 2 including the debris receiving surface ofFIG. 6 ;

FIG. 7B depicts a simulation of debris deposition in the portion of theradiation source of FIG. 7A;

FIG. 8A depicts a simulation of a first gas flow in a portion of theradiation source of FIG. 2 including the guiding device of FIG. 4A;

FIG. 8B depicts a simulation of debris deposition in the portion of theradiation source of FIG. 8A;

FIG. 9A is a simplified schematic view of an embodiment of an EUV vesselhaving a showerhead that is disposed along at least a portion of aninner vessel wall of the EUV vessel;

FIG. 9B is a simplified schematic view of an embodiment of an EUV vesselhaving a showerhead that introduces gas into the vessel via a firstplurality of nozzles and a second plurality of nozzles;

FIG. 9C is a simplified schematic view of an embodiment for an EUVvessel having a showerhead with a plurality of nozzles that arecontrolled by a common gas delivery system;

FIG. 9D is a simplified schematic view of an embodiment for an EUVvessel having a showerhead that includes a plurality of zones, whereineach zone may be separately controlled by a respective gas deliverysystem;

FIG. 10 is a simplified schematic view of an embodiment for an EUVvessel having a showerhead and an asymmetric exhaust;

FIG. 11 is a simplified schematic view of an embodiment of an EUV vesselthat is oriented at an upward leaning angle while operational;

FIG. 12A is a cross-sectional view of an embodiment for an EUV vesselthat shows a plurality of flow paths for gas being introduced into thevessel from different supplies and being exhausted by a symmetricalexhaust configuration;

FIG. 12B is a cross-sectional view of an embodiment for an EUV vesselshowing a plurality of flow paths for gas being introduced into to thevessel from various supplies and being exhausted by an asymmetricalexhaust;

FIG. 13 is a cross-sectional view of an EUV vessel having nozzles of ashowerhead distributed laterally that introduce gas into an inner vesselspace;

FIG. 14 is a cross-sectional view of an EUV vessel having a curtain flownozzle assembly for introducing gas into the vessel as curtain flows;

FIG. 15A is a cross-sectional view of an EUV vessel having a showerheadand an asymmetric exhaust that shows debris concentration within theinner vessel space, according to one simulated embodiment;

FIG. 15B is a cross-sectional view of an EUV vessel having a showerheadand an asymmetric exhaust that shows debris deposition rates on innervessel walls based on simulations, according to one simulatedembodiment;

FIG. 16A is a cross-sectional view of an EUV vessel having a curtainflow supply and an asymmetric exhaust that shows debris concentrationwithin the inner vessel space, according to one simulated embodiment;

FIG. 16B is a cross-sectional view of an EUV vessel having a curtainflow supply and an asymmetric exhaust that shows debris deposition rateson inner vessel walls, according to one simulated embodiment;

FIG. 17 is a cross-sectional view of the EUV vessel of FIG. 10comprising a guiding device;

FIG. 18A show simulated gas flow paths in the EUV vessel of FIG. 17 ;

FIG. 18B shows a simulated debris concentration in the EUV vessel ofFIG. 17 ;

FIG. 19A is a schematic view of an embodiment of the guiding device ofFIG. 17 ;

FIG. 19B is a schematic view of an embodiment of the guiding device ofFIG. 17 ;

FIG. 19C is a schematic view of a further embodiment of the guidingdevice of FIG. 17 ;

FIG. 20 is a cross-sectional view of the EUV vessel of FIG. 10comprising a further embodiment of the guiding device; and

FIG. 21 is a cross-sectional view of the EUV vessel of FIG. 17comprising a debris receiving surface;

FIG. 22A depicts a simulation of a dynamic gas flow (DGL flow) in theEUV vessel;

FIG. 22 B depicts a simulation of a dynamic gas flow (DGL flow) and twoside jet flows in the EUV vessel;

FIG. 22C is a simplified schematic view of an embodiment of an EUVvessel having two side jet flow inlets (nozzles) that introduce gas intothe EUV vessel.

DETAILED DESCRIPTION

FIG. 1 shows a lithographic system including a radiation sourceaccording to an embodiment. The lithographic system comprises theradiation source SO and a lithographic apparatus LA. The radiationsource SO is configured to generate an extreme ultraviolet (EUV)radiation beam B. The lithographic apparatus LA comprises anillumination system IL, a support structure MT configured to support apatterning device MA (e.g. a mask), a projection system PS and asubstrate table WT configured to support a substrate W. The illuminationsystem IL is configured to condition the radiation beam B before it isincident upon the patterning device MA. The projection system isconfigured to project the radiation beam B (now patterned by the maskMA) onto the substrate W. The substrate W may include previously formedpatterns. Where this is the case, the lithographic apparatus aligns thepatterned radiation beam B with a pattern previously formed on thesubstrate W.

The radiation source SO, illumination system IL, and projection systemPS may all be constructed and arranged such that they can be isolatedfrom the external environment. A gas at a pressure below atmosphericpressure (e.g. hydrogen) may be provided in the radiation source SO. Avacuum may be provided in illumination system IL and/or the projectionsystem PS. A small amount of gas (e.g. hydrogen) at a pressure wellbelow atmospheric pressure may be provided in the illumination system ILand/or the projection system PS.

The radiation source SO shown in FIG. 1 is of a type which may bereferred to as a laser produced plasma (LPP) source). A laser 1, whichmay for example be a CO₂ laser, is arranged to deposit energy via alaser beam 2 into a fuel, such as tin (Sn) which is provided from a fuelemitter 3. Although tin is referred to in the following description, anysuitable fuel may be used. The fuel may for example be in liquid form,and may for example be a metal or alloy. The fuel emitter 3 may comprisea nozzle configured to direct tin, e.g. in the form of droplets, along atrajectory towards a plasma formation region 4. The laser beam 2 isincident upon the tin at the plasma formation region 4. The depositionof laser energy into the tin creates a plasma 7 at the plasma formationregion 4. Radiation, including EUV radiation, is emitted from the plasma7 during de-excitation and recombination of ions of the plasma.

The EUV radiation is collected and focused by a near normal incidenceradiation collector 5 (sometimes referred to more generally as a normalincidence radiation collector). The collector 5 may have a multilayerstructure which is arranged to reflect EUV radiation (e.g. EUV radiationhaving a desired wavelength such as 13.5 nm). The collector 5 may havean ellipsoidal configuration, having two ellipse focal points. A firstfocal point may be at the plasma formation region 4, and a second focalpoint 6 a. The second focal point 6 a may be located at or near anintermediate focus region 6.

The laser 1 may be remote from the radiation source SO. Where this isthe case, the laser beam 2 may be passed from the laser 1 to theradiation source SO with the aid of a beam delivery system (not shown)comprising, for example, suitable directing mirrors and/or a beamexpander, and/or other optics. The laser 1 and the radiation source SOmay together be considered to be a radiation system.

Radiation that is reflected by the collector 5 forms a radiation beam B.The radiation beam B is focused at point 6 a to form an image of theplasma formation region 4, which acts as a virtual radiation source forthe illumination system IL. The point 6 a at which the radiation beam Bis focused may be referred to as the intermediate focus 6 a. Theradiation source SO is arranged such that the intermediate focus 6 a islocated at or near to an opening 8 in an enclosing structure 9 of theradiation source.

The radiation beam B passes from the radiation source SO into theillumination system IL, which is configured to condition the radiationbeam. The illumination system IL may include a facetted field mirrordevice 10 and a facetted pupil mirror device 11. The faceted fieldmirror device 10 and faceted pupil mirror device 11 together provide theradiation beam B with a desired cross-sectional shape and a desiredangular intensity distribution. The radiation beam B passes from theillumination system IL and is incident upon the patterning device MAheld by the support structure MT. The patterning device MA reflects andpatterns the radiation beam B. The illumination system IL may includeother mirrors or devices in addition to or instead of the faceted fieldmirror device 10 and faceted pupil mirror device 11.

Following reflection from the patterning device MA the patternedradiation beam B enters the projection system PS. The projection systemcomprises a plurality of mirrors 13, 14 which are configured to projectthe radiation beam B onto a substrate W held by the substrate table WT.The projection system PS may apply a reduction factor to the radiationbeam, forming an image with features that are smaller than correspondingfeatures on the patterning device MA. A reduction factor of 4 may forexample be applied. Although the projection system PS has two mirrors13, 14 in FIG. 1 , the projection system may include any number ofmirrors (e.g. six mirrors).

The radiation sources SO shown in FIG. 1 may include components whichare not illustrated. For example, a spectral filter may be provided inthe radiation source. The spectral filter may be substantiallytransmissive for EUV radiation but substantially blocking for otherwavelengths of radiation such as infrared radiation.

FIG. 2 schematically depicts an exemplary radiation source SO includinga debris mitigation system 15. For clarity purposes the laser beam 2 andthe radiation beam B are indicted by dashed lines in FIG. 2 . The debrismitigation system 15 is configured to direct a first gas flow 16 fromthe intermediate focus region 6 towards the plasma formation region 4.For example, the debris mitigation system 15 may include a first gassupply system 17. The first gas supply system 17 may be configured tosupply the first gas flow 16 towards the plasma formation region 4. Thefirst gas supply system 17 may include one or more openings 18, e.g. oneor more nozzles or slits, which are provided at the intermediate focusregion 6, e.g. at or near the intermediate focus 6. The one or moreopenings 18 may be arranged such that the first gas flow 16 towards thecollector 5 can be created. For example, the one or more openings 18 maybe arranged to direct the first gas flow 16 in a direction opposite(e.g. substantially opposite) to a propagation direction of theradiation beam B. The first gas flow 16 may have a flow rate that issufficient to reduce or prevent debris from travelling towards theintermediate focus 6 a. The first gas supply system 17 may be consideredto be or comprised in a dynamic gas lock (DGL) system. When fuel isilluminated with laser beam 2 to produce the plasma 7 a portion of thefuel may become debris. Debris may include particulate debris, such asfor example Sn clusters, Sn microparticles, Sn nanoparticles, and/or Sndeposits, molecular and/or atomic debris, such as for example Sn vapor,SnH_(x) vapor, Sn atoms, Sn ions.

The debris mitigation system 15 may be configured to direct a second gasflow 19 from the collector 5 towards the plasma formation region 4. Forexample, the debris mitigation system 15 may include a second gas supplysystem 20. The second gas supply system 20 may be configured to supplythe second gas flow 19 from the collector 5 towards the plasma formationregion 4. The second gas flow 19 may be directed towards the plasmaformation region 4 to reduce or prevent debris generated by the plasma 7from reaching the collector 5. For example, the second gas supply system20 may be arranged to supply the second gas flow 19 through a centralaperture 5 a in the collector 5. It will be appreciated that in otherexamples, the second gas supply system or a portion thereof may beprovided in the collector. For example, the second gas supply system maycomprise one or more outlets, which may be arranged within thecollector. Additionally or alternatively, the second gas supply systemmay be configured to supply the second gas flow from a perimeter portionof the collector.

The second gas flow 19 may have a flow rate that is sufficient toprevent debris from being deposited on the collector 5. For example, thesecond gas flow 19 may have a flow rate in the range of about 30 to 200slm (standard liter per minute), desirably between about 50 to 150 slm.The flow rate of the second gas flow 19 may be selected dependent on anarrangement or geometry of the second gas supply system 20.

The first gas flow 16 may be selected such as to prevent debris fromentering the illumination system IL. A flow rate of the first gas flow16 may be selected depending on a gas used in the first gas flow 16, avelocity of the gas used in the first gas flow 16, a density or pressureof the gas used in the first gas flow 16, a size of debris, e.g.particulate debris, a velocity of debris and/or a direction of debrisdiffusion in the radiation source SO. Additionally or alternatively, theflow rate of the first gas flow 16 may be selected depending on thearrangement or geometry of the first gas supply system 17. For example,the flow rate of the first gas flow 16 may be selected dependent on anumber of the openings 18, a width (e.g., diameter) of each opening 18of the first gas supply system 17 and/or a width (e.g., diameter),periphery or dimension of the intermediate focus region 6. For example,a maximum velocity of the gas used in the first gas flow 16 may be inthe range of about 1000 to 3000 m/s.

The first gas flow 16 may have a flow rate in the range of about 5 to 30slm. A flow rate of about 7 slm may be sufficient to prevent molecularand/or atomic debris generated in the radiation source SO from enteringthe illumination system IL. To suppress particulate debris from reachingthe illumination system IL, flow rates of the first gas flow larger than7 slm may be required. For example, to suppress particulate debris fromreaching the illumination system, a flow rate of larger than 15 slm ofthe first gas flow 16 may be required. At a flow rate of larger than 15slm, an asymmetric flow of the first gas flow 16 may be observed. Inother words, the first gas flow 16 may be pushed towards an internalwall of the radiation source SO, as will be described below.

FIG. 3 depicts simulated first and second gas flows 16, 19 in theradiation source SO, for the example, in which the flow rate of thefirst gas flow 16 is equal to or larger than 15 slm. From FIG. 3 , itcan be seen the first gas flow 16 is pushed against an internal wall 21(e.g., of a chamber 23) of the radiation source SO. This may be due toan interaction between the first gas flow 16 and the second gas flow 19.The interaction between the first and second gas flows 16, 19 may causethe formation of a jet of the first gas flow 16 towards a portion ofcollector 5. The formation of the jet of the first gas flow 16 mayresult in debris, e.g. particulate debris, being deposited on thecollector 5. This may lead to an increased contamination of thecollector 5 and/or the radiation source SO.

FIG. 4A schematically depicts a radiation source SO according to anembodiment. The radiation source SO depicted in FIG. 4A is similar tothat depicted in FIG. 2 , but additionally includes a guiding device.The guiding device may be provided in the form of a flow splitter 22.The first and second gas supply systems 17, 20, the one or more openings18, the laser 1, the laser beam 2 and the radiation beam B have beenomitted from FIG. 4A for clarity purposes. However, it will beappreciated that the exemplary radiation source SO depicted in FIG. 4Amay include any of the features of the radiation source SO describedabove in relation to FIGS. 1-3 . The radiation source may include achamber 23. The flow splitter 22 is arranged in the chamber 23 such thatthe first gas flow 16 is directed around the flow splitter 22. Forexample, the flow splitter 22 may be arranged such that the first gasflow is symmetrically directed around the flow splitter 22. The flowsplitter 22 may be configured to diffuse or spread, e.g. symmetricallydiffuse or spread, the first gas flow 16. By arranging the flow splitter22 in the chamber 23, recirculation of at least some of the first gasflow 16 in the chamber 23 may be reduced. This may lead to less debrisbeing deposited on the internal wall 21 of the of the radiation sourceSO. Additionally or alternatively, by arranging the flow splitter 22 inthe chamber 23 such that the first gas flow 16 is directed around theflow splitter 22, contamination of the flow splitter 22, e.g. withdebris, may be reduced or prevented.

The flow splitter 22 may be arranged in the chamber 23 of the radiationsource SO to maintain the maximum velocity of the gas used in the firstgas flow 16 at a first location in the radiation source SO. At the firstlocation the velocity of the gas used in the first gas flow 16 maycorrespond (or substantially correspond) to a maximum velocity of thegas used in the first gas flow 16, for example when no flow splitter isarranged in the chamber 23 of the radiation source SO. The flow splitter22 may be arranged in the radiation source SO to diffuse or spread thefirst gas flow 16 to prevent or reduce recirculation of some of thefirst gas flow 16, for example in a direction towards the intermediatefocus 6 a. The flow splitter 22 may be arranged in chamber 23 of theradiation source SO to diffuse or spread the first gas flow 16 at asecond location, which may be spaced or remote from the intermediatefocus point 6 a. The flow splitter 22 may be arranged in the chamber 23of the radiation source so that the maximum velocity of the gas used inthe first gas flow 16 is reduced at the second location and/or a minimumvelocity of the gas of the first gas flow 16 that may be directed in adirection away from the intermediate focus 6 a is increased.

The flow splitter 22 may be configured to reduce or prevent theinteraction between the first gas flow 16 and the second gas flow 19.The flow splitter 22 may be configured to prevent formation of a jet ofthe first gas flow 16, e.g. towards the portion of the collector 5. Thismay allow for the use of flow rates larger than 7 slm of the first gasflow 16.

Referring to FIG. 4A, the flow splitter 22 is arranged in the chamber 23to extend across a portion of the chamber 23. For example, the flowsplitter 22 may be arranged to extend at least partially along anoptical axis OA of the collector 5. In other words, the flow splitter 22may be arranged in the chamber 23 such that a central or longitudinalaxis A of the flow splitter 22 coincides with at least a part of theoptical axis OA of the collector 5. The radiation source SO may comprisea conical portion 23 a, which may be part of the chamber 23. The conicalportion 23 a may be arranged to extend from the intermediate focus 6 atowards or near the collector 5. The flow splitter 22 may be arranged inthe conical portion 23 a, for example to extend at least partially alonga central or longitudinal axis of the conical portion 23 a, which inthis example corresponds to at least a part of the optical axis OA ofthe collector 5. This may result in a symmetrical arrangement of theflow splitter 22 in the chamber 23, e.g. the conical portion 23 a. Itwill be understood that the exemplary chamber described herein is notlimited to comprising a conical portion. For example, the chamber or aportion thereof may have any suitable shape, for example, to reduce thevolume of the chamber or the portion thereof, without obstructing theradiation beam.

Referring to FIG. 4A, the flow splitter 22 is arranged at or inproximity of the intermediate focus region 6. For example, the flowsplitter 22 is arranged at or in proximity of the intermediate focusregion 6 to enable the flow splitter 22 to act on the first gas flow 16.The flow splitter 22 may be arranged at a distance from the intermediatefocus point 6 a. The distance of the flow splitter 22 from theintermediate focus point 6 a may be in the region of 5 to 15 cm.However, it should be understood that the arrangement of the flowsplitter 22 in the radiation source SO is not limited to such a distanceand other values for the distance may be selected. For example, thedistance may be selected dependent on space available at or in proximityof the intermediate focus region and/or thermal loads that may act onthe flow splitter 22, e.g. due to the radiation at the intermediatefocus region. In other words, the distance may be selected such that anythermal effects on the flow splitter 22, such as for example melting ofthe flow splitter 22, are minimized or prevented. As discussed above,the flow splitter 22 may be arranged to extend at least partially alongthe central or longitudinal axis of the conical portion 23 a, which inthis example corresponds to at least a part of the optical axis OA ofthe collector 5. This arrangement may allow the flow splitter 22 tosymmetrically direct the first gas flow 16 around the flow splitter 22,for example, to reduce or prevent the interaction between the first andsecond gas flows 16, 19 and/or may prevent the formation of a jet of thefirst gas flow 16.

The exemplary flow splitter 22 depicted in FIG. 4B is arranged to taperfrom a first 22 a end towards a second end 22 b. The first end 22 a ofthe flow splitter 22 may comprise or define an enlarged portion. Theflow splitter 22 may be arranged in the chamber 23, e.g. the conicalportion 23 a thereof, such that the first end 22 a, e.g. the enlargedportion, of the flow splitter is positioned distal from the intermediatefocus region 6. The second end 22 b of the flow splitter 22 may defineor comprise a pointed portion. The flow splitter 22 may be arranged inthe chamber 23, e.g. the conical portion 23 a thereof, such that thesecond end 22 b, e.g. the pointed portion, of the flow splitter 22 ispositioned at or proximal to the intermediate focus region 6. Theexemplary flow splitter 22 depicted in FIG. 4B comprises a conicalshape.

FIG. 4C depicts a further exemplary arrangement of the flow splitter 22.The flow splitter 22 depicted in FIG. 4C is similar to that depicted inFIG. 4B. The first end 22 a of the flow splitter 22 defines or comprisesthe enlarged portion. The second end 22 b of the flow splitter 22comprises or defines a rounded portion. The exemplary flow splitterdepicted in FIG. 4C may be considered as comprising a substantiallytruncated conical shape. It should be understood that the flow splitterdisclosed herein is not limited to a conical or truncated conical shape.In other examples, the flow splitter may comprise a conical or truncatedconical shape having one or more flat portions. Alternatively, the flowsplitter may comprise a spiral or helical shape.

Referring to FIGS. 4B and 4C, an extension or dimension of the flowsplitter 22, for example along the longitudinal or central axis A of theflow splitter 22, may be selected depending on a dimension, volumeand/or shape of the chamber 23 of the radiation source SO. The extensionor dimension of the flow splitter 22 may be selected such that the flowsplitter 22 interacts with the first gas flow 16 and/or the flowsplitter directs the first gas flow around the flow splitter 22, asdescribed above, e.g. when the flow splitter 22 is arranged in thechamber 23 of the radiation source SO. An exemplary extension ordimension of the flow splitter 22 along the longitudinal or central axisA of the flow splitter 22 may comprise about 3 to 30 cm, e.g. 10 to 20cm. However, it should be understood that the exemplary flow splitterdisclosed herein is not limited to such an extension or dimension.

The radiation source SO may include a heating element 24, which may bepart or comprised in the flow splitter 22. The heating element 24 may beconfigured to increase a temperature of the flow splitter 22, forexample to increase an amount of the first gas flow 16 that is directedaround the flow splitter 22.

The heating element 24 may be configured to increase the temperature ofthe flow splitter 22 to or above a first temperature at which anincreased amount of the first gas flow is directed around the flowsplitter 22. For example, an increase of the temperature of the flowsplitter 22 to or above the first temperature may result in an increaseof the velocity of at least some of the atoms of the first gas flow 16,e.g. when at least a portion of the first gas flow 16 comes into contactwith the flow splitter 22. An increase of the temperature of the flowsplitter 22 to or above the first temperature may cause heat to betransferred to a portion of the first gas flow 16 that comes intocontact with the flow splitter 22. The transfer of heat to the portionof the first gas flow 16 may cause the gas of the portion of the firstgas flow to expand and/or a viscosity of the gas of the portion of thefirst gas flow to increase. In other words, the gas of the portion ofthe first gas flow that comes into contact with the flow splitter 22 maycomprise an increased viscosity. The gas of the portion of the first gasflow 16 comprising the increased viscosity may act on another portion ofthe first gas flow, which is incident on the flow splitter 22 and/orcause the other portion of the first gas flow 16 to be directed aroundthe flow splitter 22. In other words, due to the increased viscosity ofthe gas of the portion of the first gas flow 16, the effective dimensionof the flow splitter 22 may be considered as being increased relative tothe actual dimension of the flow splitter 22.

The first temperature may be equal to or larger than the meltingtemperature of the fuel used to create the plasma 7. In other words, thefirst temperature may be selected dependent on the fuel used to createthe plasma 7. For example, when tin is used as a fuel, the heatingelement 24 may be configured to increase the temperature of the flowsplitter to a temperature of about or larger than 230° C. (which largelycorresponds to the melting temperature of tin). For temperatures below200° C., any fuel, e.g. tin, deposited on the flow splitter 22 may besolid. The solid fuel may cause diffraction or block at least a portionof the radiation beam B directed towards the intermediated focus 6 a.

The heating element 24 may be configured to maintain the temperature ofthe flow splitter 22 below a second temperature. At or above the secondtemperature diffusion of debris that may be present on the flow splitteroccurs or increases. At the second temperature or above the secondtemperature, diffusion of debris that may be present on the flowsplitter 22 may be increased. For example, the diffusion coefficient oftin vapor in a hydrogen atmosphere may increase with increasingtemperature. By maintaining the temperature of the flow splitter 22below the second temperature, diffusion of debris in the chamber 23 maybe reduced. The amount of debris that may be present on the flowsplitter 22 is considered to be small, for example, due to the flowsplitter 22 being arranged in the chamber 23 to direct the first gasflow 16 around the flow splitter 22 and/or due to the use of flow rateslarger than 7 slm of the first gas flow 16, as described above.

The heating element 24 may be embedded in the flow splitter 22. It willbe appreciated that in other embodiments, the heating element may beprovided separately. In such embodiments, the heating element may bearranged to increase the temperature of the flow splitter. The heatingelement 24 may be provided in the form of a resistive heating element.It will be appreciated that in other embodiments, the flow splitter 22may be inductively heated and/or the heating element may be provided inthe form of an electromagnetic element, e.g. a coil or the like. Anelectronic oscillator, e.g. a radio frequency generator, may be providedto generate electric currents in the electromagnetic element, which mayresult in heat being generated in the electromagnetic element.

Referring to FIGS. 4A and 4D, in some embodiments, the flow splitter 22may be configured for cooling by a coolant. The flow splitter 22 may becooled, for example to reduce the thermal loads that may act on the flowsplitter 22, e.g. due to the radiation at the intermediate focus region.The flow splitter 22 may be cooled to maintain a temperature of the flowsplitter 22 below a melting temperature of the fuel used to create theplasma 7. This may prevent distribution/diffusion of liquid fuel thatmay be present on the flow splitter 22 onto the internal wall 21 or anyother component of the radiation source SO. As described above, theamount of debris that may be present on the flow splitter 22 isconsidered to be small, for example, due to the flow splitter 22 beingarranged in the chamber 23 to direct the first gas flow 16 around theflow splitter 22 and/or due to the use of flow rates larger than 7 slmof the first gas flow 16, as described above.

The coolant may be supplied by a coolant source 25. For example, theflow splitter 22 may comprise a channel 26 to receive the coolant fromthe coolant source 25 and/or to flow the coolant through the flowsplitter 22. The flow splitter 22 may be configured for connection tothe coolant source 25. The coolant source 25 may be configured to supplythe flow splitter 22 with a coolant. For example, the coolant source 25may be configured to supply the flow splitter 22 with a coolant todecrease a temperature of the flow splitter 22, e.g. below a meltingtemperature of the fuel used to create the plasma 7 and/or the secondtemperature, as described above. The coolant may be provided in the formof a coolant fluid, e.g. a coolant liquid or a coolant/cold gas etc. Itwill be appreciated that the flow splitter may be configured for beingcooled by the coolant instead to or addition to comprising the heatingelement 24.

FIG. 5 schematically depicts a further embodiment of the radiationsource SO. The radiation source SO depicted in FIG. 5 is similar to thatdepicted in FIG. 4A. The first and second gas supply systems 17, 20, theone or more openings 18, laser 1, laser beam 2 and the radiation beam Bhave been omitted from FIG. 5 for clarity purposes. However, it will beappreciated that the exemplary radiation source SO depicted in FIG. 5may include any of the features of the radiation source SO describedabove in relation to FIGS. 1-4 .

The exemplary flow splitter 22 of the radiation source SO depicted inFIG. 5 includes a plurality of further openings 27, which may beprovided in the form of nozzles or slits. The plurality of furtheropenings 27 (or each further opening of the plurality of furtheropenings 27) may be configured to direct a third gas flow 28 towards thecollector 5. The third gas flow may comprise a flow rate in the range ofabout 1 to 50 slm. The plurality of further openings 27 may be arrangedon the flow splitter 22 such that the third gas flow 28 from theplurality of further openings 27 interacts with the first gas flow 16.The interaction between the first gas flow 16 and the third gas flow 28may to direct or push the first gas flow 16 into proximity with theinternal wall 21 of the chamber 23, e.g. the conical portion 23 a. Theprovision of the plurality of further openings 27 for directing thethird gas flow 28 towards the collector 5 may lead to an increasedspreading of the first gas flow 16. The increased spreading the firstgas flow 16 may result in a reduced or suppressed interaction betweenthe first and second gas flows 16, 19.

The plurality of further openings 27 may be circumferentially,peripherally and/or axially arranged on the flow splitter 22. In otherwords, the plurality of further openings 27 may be arranged to extendaround the flow splitter 22 and/or in a direction of the central orlongitudinal axis A of the flow splitter 22. The plurality of furtheropenings 27 may be symmetrically arranged on the flow splitter 22, forexample to cause a symmetric flow of the first gas flow 16 and/or thethird gas flow 28 around the flow splitter 22.

The exemplary first gas supply system 17 depicted in FIG. 2 may beconfigured to supply the third gas flow 28 to the flow splitter 22. Forexample, the flow splitter 22 may be connected or connectable to thefirst gas supply system 17 e.g. to enable supply of the third gas flow28 to the flow splitter 22. It will be appreciated that in a furtherexample, the debris mitigation system may comprise a further gas supplysystem, which may be configured to supply the third gas flow to the flowsplitter. The flow splitter may be connected or connectable to thefurther gas supply system, e.g. to enable supply of the third gas flowto the flow splitter. Although the flow splitter 22 depicted in FIG. 5comprises a plurality of further openings 27, it will be appreciatedthat in other embodiments the flow splitter may comprise a singlefurther opening, which may be configured to direct the third gas flowtowards the collector.

FIG. 6 schematically depicts a further embodiment of the radiationsource SO. The radiation source SO depicted in FIG. 6 is similar to thatdepicted in FIG. 4A. The first and second gas supply systems 17, 20, theone or more openings 18, the laser 1, the laser beam 2 and the radiationbeam B have been omitted from FIG. 4A for clarity purposes. However, itwill be appreciated that the exemplary radiation source SO depicted inFIG. 6 may include any of the features of the radiation source describedabove in relation to FIGS. 1-5 .

The exemplary radiation source SO depicted in FIG. 6 comprises a debrisreceiving surface 29 a, which may be part of or provided by a bar orobscuration bar 29. The bar 29 may be arranged in the chamber 23, e.g.the conical portion 23 a, to prevent debris from reaching theintermediate focus region 6. The bar 29 may be arranged to intersect orextend across the optical axis OA of the collector 5. In thisarrangement, the bar 29 can be considered to obscure the direct line ofsight of debris, which may include ballistic particulate debris, and/orof a portion of the laser beam 2, e.g. the portion of the laser beam 2that passes through the plasma formation region 4. In other words, thebar 29 may be configured to reflect the portion of the laser beam 2 awayfrom the intermediate focus region 6 of the radiation source SO.

In the exemplary radiation source depicted in FIG. 6 , the flow splitter22 is arranged between the bar 29 and the intermediate focus region 6.In this arrangement, the bar 29 is arranged to extend over or overlapwith at least a portion or all of the flow splitter 22. For example, thebar 29 may be arranged to extend over or overlap with the enlargedportion of the first end 22 a of the flow splitter 22 so that debrisgenerated by the plasma 7 is incident on the debris receiving surface 29a of the bar 29. In other words, flow splitter 22 may be arranged in theshadow of the bar 29.

Although in the exemplary radiation source SO depicted in FIG. 6 , thedebris receiving surface 29 a was described as being part of the bar 29,it will be appreciated that in other embodiments of the radiationsource, such as for example any of those described in relation to FIGS.4A, 4B, 4C and 5 , the debris receiving surface 29 a may be provided byor be part of the flow splitter 22. In such embodiments, the flowsplitter 22 may comprise any of the features of the bar 29, describedherein. Additionally, the flow splitter 22 may be configured such thatthe flow splitter 22 is able to withstand the heat or heat/thermal loadcreated by the plasma 7 or that of the radiation at the intermediatefocus region 6. The flow splitter 22 may be configured to reflect theportion of the laser beam 2 that passes through the plasma formationregion 4, away from the intermediate focus region 6. For example, whenthe debris receiving surface 29 a is provided by the flow splitter 22,the extension or dimension of the flow splitter 22, e.g. in a directionperpendicular and/or parallel to the central or longitudinal axis A, ofthe flow splitter 22, may be increased relative to the extension ordimension, e.g. in a direction perpendicular and/or parallel to thecentral or longitudinal axis A, of a flow splitter 22 that is used incombination with the bar 29.

FIG. 7A depicts a simulation of the first gas flow 16 in a radiationsource SO with no flow splitter present. The radiation source SOdepicted in FIG. 7A comprises the bar or obscuration bar 29 describedabove in relation to FIG. 6 , which is arranged in the chamber 23, e.g.the conical portion 23 a. The first gas flow 16 may be considered to besubstantially laminar in the radiation source SO. However, it can beseen in FIG. 7A that some of the first gas flow 16 recirculates. Therecirculation of some of the first gas flow 16 may be, for example dueto a jet being formed from some of the gas of the first gas flow 16,which may interact with adjacent gas, e.g. to pull the adjacent gasalong with the same velocity or speed. Fresh gas of the first gas flow16 may flow with a lower velocity or speed along the internal wall 21 toprevent gas depletion or the formation of an under pressure. Therecirculation of some of the first gas flow 16 may result in debrisbeing deposited on the internal wall 21 of the radiation source SO, e.g.the chamber 23.

FIG. 7B depicts a simulation of surface deposition of debris, e.g.atomic tin debris, in a radiation source with no flow splitter present.It can be seen that debris is distributed in the chamber 23, e.g. theconical portion 23 a, and extends in a direction towards theintermediate focus region 6.

FIG. 8A depicts a simulation of the first gas flow 16 in a radiationsource SO comprising the flow splitter 22 described above. It can beseen from FIG. 8B that by arranging the flow splitter 22 in the chamber23, e.g. the conical portion 23 a, recirculation of the first gas flow16 is reduced. This results in a reduced debris deposition in thechamber 23, e.g. the conical portion 23 a, as depicted in FIG. 8B, whichdepicts a simulation of surface deposition of debris, e.g. atomic tindebris, in a radiation source comprising the flow splitter 22. In otherwords, by arranging the flow splitter 22 in the chamber 23, e.g. theconical portion 23 a, the extension of debris towards the intermediatefocus region 6 is reduced.

The first, second and/or third gas flow may comprise hydrogen gas. Itwill be appreciated that in other embodiments another gas or a mixtureof gases may be used. For example, in other embodiments, the first,second and/or third gas flow may comprise argon or helium gas.

The material of the flow splitter 22 may be selected to be corrosionresistant, e.g. to be resistant against corrosion by the fuel in theenvironment in the radiation source SO, e.g. the hydrogen environment inthe radiation source SO. The material of the flow splitter 22 may beselected to be resistant to the thermal loads acting on the flowsplitter, e.g. due to the radiation in the radiation source SO and/orthe plasma 7, and/or the increase of the temperature of the flowsplitter 22 to or above the first temperature, as described above. Theexemplary flow splitter 22 may comprise or be made of a metal or metalalloy. For example, the material of flow splitter may be or comprisemolybdenum, tungsten, aluminum, stainless steel, copper or an alloythereof. The flow splitter 22 may comprise a metal or metal alloysurface. The metal or metal alloy surface of the flow splitter may leadto an improved recombination of hydrogen radials, which may be presentin the radiation source SO. For example, hydrogen (H₂) molecules maysplit into hydrogen radicals due to their absorption of heat and/orradiation or ion collisions. The hydrogen radicals may be beneficial forremoving debris, e.g. tin, from the internal wall 21 of the radiationsource. The presence of hydrogen radicals may cause contamination in thechamber 23, such as spitting of fuel, for example, when the hydrogenradicals diffuse into layers of fuel in the chamber 23 that are liquid.By providing the flow splitter with a metal or metal alloy surface, therecombination of hydrogen radicals may be improved and/or thecontamination, e.g. spitting of fuel, in the chamber 23 reduced. It willbe appreciated that in other embodiments the flow splitter may compriseanother material, such as for example a ceramic material. The ceramicmaterial may comprise a silicon dioxide, zirconium nitride, or zirconiumoxide material.

As described above, a debris mitigation system is configured to direct afirst gas flow from the intermediate focus region towards the plasmaformation region. For example, the debris mitigation system may includea first gas supply system. The first gas supply system may be configuredto supply the first gas flow towards the plasma formation region. Thefirst gas supply system may include one or more openings, e.g. one ormore nozzles or slits, which are provided at the intermediate focusregion, e.g. at or near the intermediate focus. The one or more openingsmay be arranged such that the first gas flows towards the collector. Forexample, the one or more openings may be arranged to direct the firstgas flow in a direction opposite (e.g. substantially opposite) to apropagation direction of the EUV radiation beam. The first gas flow mayhave a flow rate that is sufficient to reduce or prevent debris fromtravelling towards the intermediate focus point 127 a. The first gassupply system may be considered to be or comprised in a dynamic gas lock(DGL) system.

As shown in FIG. 22A, to protect at least part of the EUV lithographicapparatus from fuel contamination from the EUV source, a first gas flowis introduced at the intermediate focus (IF) region 127 to push debrisparticles created at plasma formation back into the source vessel (thefirst gas flow depicted in FIG. 22A is similar to the first gas flow 16in FIG. 7A). The first gas flow, herein referred to as a dynamic gasflow or dynamic gas lock (DGL) flow, can be created by one or moreconverging gas inlets. High velocity gas jets may be provided forexample from two arrays of several gas inlets which converge towards theoptical axis of the EUV collector (as shown for example in FIGS. 9 and10 ), forming a single high speed first gas flow in the EUV vessel 100.The high speed first gas flow creates a drag force to drag the debrisparticles away from the IF region 127 and at the same time provides aPeclet type protection of at least part of the lithographic apparatusagainst fuel vapor and/or derivatives of the fuel (e.g., tin hydride).Experimental results show however that with a dynamic gas flow (e.g.first gas flow) having a gas flow rate of less than 10 slm, the amountof debris particles passing through the IF point 127 a may be an orderof magnitude higher than the desired cleanliness specification for anEUV source.

To solve this problem, a higher speed first gas flow could be used toprotect at least part of the lithographic apparatus from fuelcontamination. However, at higher speed of the first gas flow, thecollector contamination and fuel droplet stability (when emitted by thefuel generator) may get worse. It is assumed that the high speed, narrowdynamic gas flow may interact with a collector cone flow provided froman opposite direction and thereby still reach the collector, causingfuel droplet instability and/or additional collector contamination. Thehigh speed first gas flow may introduce recirculation and also bring thedebris particles closer to the IF region 127 such that they can passinto the lithographic apparatus through the intermediate focus point 127a, thereby at least partially contradicting the debris mitigationfunction of the dynamic (first) gas flow. The gas recirculation mayoccur especially in the vessel top, thereby acting as a “transport belt”for debris towards the IF region 127.

In an embodiment it is proposed to use a second gas supply systemarranged to provide a second gas flow which confines the first gas flowrecirculation to the top of the EUV vessel 100 in the intermediate focusregion 127. The second gas supply system comprises one or more openingsarranged to direct the second gas flow preferably in a directionsubstantially perpendicular to the propagation direction of the firstgas flow (i.e. substantially perpendicular to the optical axis of theEUV collector). In an embodiment shown in FIG. 22B, a pair of inlets 164a, 164 b to provide two counter gas flow jets (i.e. a second gas flow)are arranged in the proximity of the intermediate focus region 127,downstream of the first gas flow. Preferably the second gas flow issupplied with a gas flow rate of at least 2 slm per inlet (i.e. at least4 slm per pair), even more preferably at least 5 slm per jet. The secondgas flow may have substantially similar or even higher speed than thefirst gas flow.

The squeeze jets work best with a straight DGL flow (i.e. aligned withthe optical axis of the EUV vessel). The gas jet inlets 164 a, 164 b maybe aligned to provide a centered flow (as shown in FIG. 22B). The gasjet inlets may also be slightly offset. An offset between the squeezejets may improve robustness towards tilt (as it creates a largerinteraction region at the interception of the gas flows), as long as theDGL flow still remains substantially straight. Alternatively, the gasjet inlets 164 a, 164 b are arranged to provide a tilted second gas flowwhich forms an angle with the direction of the first gas flow (notrepresented).

The pair of counter gas jet inlets 164 a, 164 b may be arranged in theIF region 127 close to the first gas inlet at the IF point 127 a forbetter performance (as shown in FIG. 22 C), provided there is nomechanical design constraint. A function of the pair of counter gas jets164 a, 164 b is to squeeze and diffuse the dynamic gas flow by means ofmomentum exchange. A similar principle is used in liquid atomizers,where two high velocity side gas jets break a liquid stream into microdroplets. The side gas jets 164 a, 164 b slow down and diffuse thenarrow but high velocity dynamic gas flow jet. The large scalerecirculation close to a vessel wall is thereby eliminated or reduced,and instead a substantially unidirectional gas flow field is establishedtowards the exhaust, thereby helping to protect the EUV vessel 100 fromfuel contamination.

The squeeze jet inlets 164 a, 164 b are arranged such that the gas jetflows are substantially orthogonal to the main direction of the dynamicgas flow, whereas the dynamic gas flow nozzles are arrangedsubstantially aligned with the resulting dynamic gas flow jet. Thevelocity of the squeeze jets may be tuned such that is substantiallysimilar to the velocity of the dynamic gas flow jet near theirinterception point 166. Without being bound to a theory, it is expectedthat the momentum of the squeeze jets preferably is substantiallycomparable in value to the momentum of the dynamic gas flow jet at theirinterception point 166. The inlets and nozzles size, shape (round,racetrack, squared etc) and the spacing of the squeeze jet inlets 164 a,164 b to the IF point 127 a may be varied to enhance the debrismitigation of the EUV vessel 100.

One or more benefits of introducing side jets are: it allows a high DGLflow for protection of at least part of a lithographic apparatusprotection; it may reduce or even eliminate recirculation from exhaustto lower cone; it may provide a positive Peclet protection in the IFcone; and it can significantly reduce the dynamic gas flow speed toeliminate the negative impact on collector and fuel droplets stabilitywithout sacrificing the function of dynamic gas flow.

Although FIGS. 22B and 22C depict a pair of squeeze jets, it is alsopossible to use a single squeeze jet or multiple squeeze jets (i.e. 2 ormore squeeze jets). Simulations show that two squeeze jets or multiplepairs of gas flow squeeze jets provide advantageous results; however,depending on the specific conditions of the total gas flow in the EUVvessel also asymmetric second gas flow arrangements may be useful. In anasymmetric flow design, the momentum of the squeeze jets is stillsubstantially similar in value to the momentum of the dynamic gas flowat their interception point.

According to an aspect of the invention, combinations of the aboveembodiment with other gas flow supplies are also possible. For example,the EUV vessel may also comprise (i.e. in addition to the first andsecond gas flows) a third gas flow in the form of a showerhead disposedalong at least a portion of the inner vessel wall. The showerheadincludes a single or a plurality of nozzles configured to introduce gasinto the vessel. The showerhead has at least one inlet configured tosupply the gas into the showerhead. One or more exhausts may beconfigured to remove gas introduced into the vessel, the one or moreexhausts being oriented along at least a portion of the inner vesselwall so that the gas is caused to flow away from the EUV collector. Atleast one exhaust may be disposed along the inner vessel wall at anazimuthally asymmetric position and configured to exhaust gas from thevessel. The squeeze jets or a guiding device can also work well togetherwith a curtain flow (for example with a curtain flow 122 as depicted inFIG. 18A). Combining a DGL flow with a second gas flow (squeeze jets), acurtain flow and a showerhead flow in the lower part of the EUV vesselis also possible. Another embodiment is a curtain flow substantiallyperpendicular to the first gas flow which is provided by the guidingdevice (as depicted for example in FIG. 20 ). It is possible to use thisin combination with the squeeze jets or stand-alone (no jets).

Furthermore, a guiding device may be added in the EUV chamber such thatthe first gas flow is directed around the guiding device. The guidingdevice may be introduced in the path of the first gas flow before orafter the interception point with the second gas flow.

A radiation source for use in a lithographic system may be provided inthe form of a laser produced plasma (LPP) source (or simply, “source”)The radiation source generates extreme ultraviolet radiation (EUV)radiation by producing a plasma from a fuel such as tin (Sn) in a plasmavessel. The radiation source may comprise a EUV vessel. In some cases,tin plasma is produced by illuminating droplets of liquid fuel with ahigh energy laser radiation. Although tin is referred to in thefollowing description, any suitable fuel may be used. EUV photonsemitted from the plasma are collected by a near normal incidenceradiation collector (sometimes referred to more generally as a normalincidence radiation collector), which may be provided in the form of aEUV collector, within the vessel and transmitted to an intermediatefocus point where they enter at least part of a lithographic apparatus.In many cases, tin debris will be generated and remain inside the vesselas a result of illuminating tin matter with laser radiation.

Tin debris may include any tin matter or tin product that remains in thevessel after having been illuminated or intended to be illuminated bylaser radiation. Tin debris can include, for example, ionic tin, tinvapor, tin microparticles, tin products (SnH₄ gas), or tin deposition.In many cases, tin debris becomes deposited on inner vessel walls of theEUV vessel as well as on the EUV collector. Once deposited, tin debriscan spit, drip, and drop onto other surfaces within the vessel. As aresult, tin debris can accumulate to an extent that it reduces the EUVcollector's reflectivity or otherwise blocks EUV optical paths. Thisultimately leads to reduced collector lifetime and source availability.

In some cases, certain surfaces (e.g., vessel walls, vanes, andscrubbers) are kept cold during operation of the source. This mayeliminate a portion of tin dripping and spitting by keeping the tindebris in a solid state. However, in certain circumstances, tin maystill accumulate to an extent that it drops due to gravitational forcesand gas pressure on the EUV collector while the EUV vessel isoperational. Further, Sn accumulation on these cold surfaces may lead toEUV path blockage as well as disturbances to gas flows for sourceoperation.

In other cases, certain surfaces are kept at a temperature above amelting point of Sn. While this may reduce Sn accumulation and EUV pathblockage as a result of maintaining tin debris in a liquid state, liquidstate tin is prone to spitting and dripping as mentioned previously.

Certain sources address Sn debris and deposition on vessel surfaces byproviding gas supplies at locations within the vessel to flush tindebris out of the vessel. One of these is a center supply thatintroduces gas into the vessel near a center of the EUV collector. Inaddition, certain sources may have a perimeter supply that introducesgas into the vessel at locations proximate to a perimeter of the EUVcollector. The center supply and the perimeter supply provide gas flowpaths that help protect the EUV collector from Sn debris to some extentby providing barriers to diffusion against Sn debris, as well asenergetically favourable flow directions away from the EUV collector.

While embodiments that have a center supply and a perimeter supplyenable a level of protection of the EUV collector, Sn debris may stillaccumulate on the inner vessel walls to an extent that it blocks EUVoptical paths. Additionally, if Sn debris accumulates on a region of theinner vessel wall that is gravitationally above the EUV collector, theSn debris may drop down onto the EUV collector. As a result, it would bebeneficial to have gas sources for introducing gas into the vessel inaddition to the center supply and the perimeter supply to allow forprotection of one or more inner vessel walls.

In certain embodiments, an EUV vessel may include a vessel wall supplyto introduce gas into the vessel. According to some embodiments, thevessel wall supply may include a showerhead disposed along at least aportion of the inner vessel wall, the showerhead having a plurality ofnozzles to introduce gas into the vessel. According to theseembodiments, the showerhead may be of a similar shape as the innervessel wall. Thus, if, for example, the inner vessel wall has a conicalshape, the showerhead may be of a conical shape as well. Likewise, ifthe inner vessel happens to have a cylindrical, rectangular, or otherpolyhedral shape, the showerhead that is included with the EUV vesselmay similarly have a cylindrical, rectangular, or other polyhedralshape, respectively. According to further embodiments, the showerheadmay have a shape that is different from the inner vessel wall. In someembodiments, gas that is supplied into the vessel via the vessel wallsupply, the perimeter supply, or the center supply may include hydrogengas.

FIG. 9A is a simplified schematic view of an embodiment of an EUV vessel100 having a showerhead 101 that is disposed along at least a portion ofan inner vessel wall 104 of the EUV vessel 100. According to theembodiment, a laser radiation pulse 105 is shown to illuminate a targetmaterial 111 within a material target region (not shown). Plasma 107 isshown to result, which, for example, may produce EUV radiation 115. TheEUV radiation 115 is shown to be reflected by an EUV collector 102toward an intermediate focus (IF) region 157.

According to the embodiment of FIG. 9A, the showerhead 101 is shown toinclude a plurality of nozzles 120 that are disposed along at least aportion of the inner vessel wall 104. Each of the plurality of nozzles120 are shown to enable a flow 140 of gas into the vessel in a directionthat is away from the inner vessel wall 104 along which they aredisposed. Although not shown, it is to be understood that nozzles 120may also be included on the right-most portion of the inner vessel wall104. That is, the showerhead 101 may extend along the right side of theinner vessel wall 104, including the portions of the inner vessel wall104 that are proximate to the IF region 157.

FIG. 9A shows that the inner vessel wall 104 can be defined by arectangular cross-section 151. The inner vessel wall 104 may have othercross-sectional shapes, which can be, for example, circular,ellipsoidal, a non-rectangular polygonal cross section, or differentcross-sectional shapes at different distances along a length of theinner vessel wall 104. It is to be understood that any number of innervessel wall 104 shapes may be implemented without departing from thescope and spirit of the embodiments described herein. Thus, by way ofexample, various non-limiting examples of conical shaped inner vesselwalls 104 will be described below.

FIG. 9B is a simplified schematic view of an embodiment of an EUV vessel100 (or simply ‘vessel’) having a showerhead 101 that introduces gasinto the vessel 100 via a first and second plurality of nozzles 120 aand 120 b. In the embodiment shown, the EUV vessel 100 is shown toinclude an inner vessel wall 104 having a conical shape. The showerhead101 is shown to take on a similar shape as a result of being disposedalong at least a portion of the inner vessel wall 104. The EUV vessel100 is further shown to be able to receive a laser radiation pulse 105that enters the vessel via a center region 109. The laser radiationpulse 105 is shown to become incident on a target material 111 within atarget material region (not shown). Plasma 107 is shown to result, whichgives off plasma emission 113. Some of the plasma emission, whichincludes EUV radiation 115, is shown to reflect off of the EUV collector102, travel through the vessel 100, and enter at least part of alithographic apparatus 117.

Also shown in FIG. 9B are a center supply 106 and a perimeter supply108, both of which introduce gas into the vessel 100 at locationsproximate to the EUV collector 102. Both of the center supply 106 andthe perimeter supply 108 may each include a plurality of gas inlets tointroduce gas into the vessel 100. The center supply 106 and theperimeter supply 108 introduce gas in a way that lowers Sn debriscontact instances with the EUV collector 102. For example, the centersupply 106 and perimeter supply 108 provide diffusion barriers againstSn vapor or microparticles and energetically favorable flow paths thatare in a direction away from the EUV collector 102 for the same. As aresult, the EUV collector 102 is provided with some degree of protectionfrom Sn debris.

FIG. 9B also shows a showerhead 101 that introduces gas into the vessel100 away from an inner vessel wall 104. In the embodiment shown, theshowerhead 101 includes a first supply 103 a and a second supply 103 b.The first supply 103 a is shown to supply a first plurality of nozzles120 a that introduces gas into the vessel 100 as a first plurality offlows 140 a. The second supply 103 b is shown to supply a secondplurality of nozzles 120 b that introduces gas into the vessel 100 as asecond plurality of flows 140 b. The first supply 103 a and the secondsupply 103 b of the showerhead 101 are shown to be able to separatelycontrol the supply of gas to the first plurality of nozzles 120 a andthe second plurality of nozzles 120 b, respectively. Additionally, thesecond plurality of nozzles 120 b is shown to be more proximal to theEUV collector 102, whereas the first plurality of nozzles 120 a is shownto be more proximal to an IF region 157 of the vessel that is proximateto the at least part of the lithographic apparatus 117.

In certain embodiments, the first plurality of nozzles 120 a and thesecond plurality of nozzles 120 b may be separately supplied withseparately controlled gas delivery systems (not shown). In theseembodiments, having separately controlled gas delivery systems for thefirst plurality of nozzles 120 a and the second plurality of nozzles 120b may enable control over flow geometries or flow paths that occurwithin the vessel 100 that result from introducing gas into the vessel100 via the showerhead 101. More on controlling flow geometries withinthe vessel 100 will be discussed herein. In certain embodiments, thefirst plurality of nozzles 120 a and the second plurality of nozzles 120b may be considered individual zones for introducing gas into the vessel100.

The first plurality of flows 140 a and the second plurality of flows 140b are shown to be in a direction that is away from the inner vessel wall104. As a result of the directionality of the first and second pluralityof flows 140 a and 140 b, flow geometries may be produced within thevessel 100 that prevent deposit of Sn debris onto surfaces of innervessel wall 104. For example, gas that is introduced via the first andsecond plurality of nozzles 120 a and 120 b may provide a diffusionbarrier that may suppress Sn vapor flux, SnH₄ flux, and other Sn debrisflux onto the inner vessel wall 104.

In certain embodiments, there may be a hydrogen radical flux onto theinner vessel wall 104. Generally speaking, hydrogen radicals may bepresent within the vessel 100 as a result of the plasma 107, as well asEUV radiation 115 absorption by hydrogen gas. Hydrogen radicals may bebeneficial in certain circumstances involving ‘cold’ walls (portions ofthe inner vessel wall 104 that are conditioned to be below a meltingpoint of Sn), for example, in which they may remove solid Sn depositsfrom the inner vessel wall 104 by forming SnH₄ gas. Under thesecircumstances and according to certain embodiments, hydrogen radicalsmay be additionally supplied to help a removal of solid Sn deposits fromthe inner vessel wall 104, for example, by supplying hydrogen radicalsthrough the showerhead 101, the center supply 106, the perimeter supply108, or the dynamic gas lock (DGL) supply 110. In other embodimentsinvolving warm regions of the inner vessel wall 104 that may includeliquid Sn, hydrogen radical flux onto the warm regions may be reduced.

In the various embodiments, the inner vessel wall 104 may be defined bya separate wall interface which enables the first and second pluralityof nozzles 120 a and 120 b to direct gas flows into the EUV vessel 100.In these configurations, the showerhead 101 is primarily behind theinner vessel wall 104. In other embodiments, the showerhead 101 itselfhas an inner surface that will define the inner vessel wall 104. Incertain other embodiments, each nozzle of the first and second pluralityof nozzles 120 a and 120 b may be provided with an individual gas linefor introducing gas into the vessel 100. In still other embodiments, theshowerhead 101 may be integrated into the inner vessel wall 104 suchthat lines that supply each of the nozzles of the first and secondplurality of nozzles 120 a and 120 b are hogged out from a first piecethat is then mated with a second piece, the first or second piecedefining the inner vessel wall 104.

Although the first and second plurality of flows 140 a and 140 b areshown to introduce gas in a direction that is orthogonal to the innervessel wall 104, it should be appreciated that there are many ways inwhich to introduce gas into the vessel 100 that may not be orthogonal tothe inner vessel wall 104 but which still fall within the spirit andscope of the embodiments. Moreover, while each of the first and secondplurality of nozzles 120 a and 120 b are shown to have a similardirectionality, there may any number of variations to the individualdirectionalities of individual nozzles that may be implemented withoutdeparting from the substance and scope of the embodiments described. Forexample, certain embodiments may implement the plurality of nozzles atsimilar directions or angles relative to inner vessel wall 104, whileother embodiments may implement nozzles having angles that are differentfrom one another relative to the inner vessel wall 104 to suit the needsof different embodiments of EUV sources or EUV vessels.

It is also to be understood that the showerhead 101 in FIG. 9B extendsalong a perimeter (e.g., a circumference) of the of the inner vesselwall 104 such that the first and second plurality of nozzles 120 a and120 b may be configured to introduce gas into the vessel 100 along theentire perimeter of the inner vessel wall 104. In other embodiments, theshowerhead 101 may not necessarily extend the entire perimeter of theinner vessel wall 104, or may do so but only for a certain length alongthe inner vessel wall 104, laterally. In other words, embodiments mayhave any one of a number of patterns for distributing the first andsecond plurality of nozzles 120 a and 120 b along the inner vessel wall104 of the vessel 100 to suit the needs of different embodiments. Forexample, in certain embodiments, a first or second plurality of nozzles120 a or 120 b may be located in a region that is gravitationally abovethe EUV collector 102 to prevent Sn debris from depositing on the vesselwall in the region and subsequently dropping down onto the EUV collector102 while the EUV source is operational.

While inner vessel wall 104 is shown to include smooth surfaces, itshould be understood that inner vessel wall 104 may include vanedsurfaces (e.g., surface with vanes or surfaces defined by vanes). Forexample, in certain embodiments having one or more inner vessel walls104 that include vanes, gas may be introduced via the back of the vanesand released into the vessel 100 through openings in a plasma-facingsurface of the vanes. Thus, appropriate channels (not shown) inside thevanes may be used to deliver gas to nozzles that are integrated into thevanes in a pattern desired. As a result, the showerhead 101 may beintegrated into vane surfaces of vessel 100 to achieve flow geometriesof gas that reduce contamination of the one or more inner vessel walls104. In other embodiments having one or more inner vessel walls 104 thatinclude vanes, gas may be introduced from valleys between adjacentvanes.

Also shown in FIG. 9B is symmetric exhaust 112, which exhausts gas outof the vessel 100. The symmetric exhaust 112 may be arrangedsymmetrically around a perimeter of the inner vessel wall orasymmetrically. In one embodiment, the symmetric exhaust 112 may includea single exhaust that extends around a full perimeter of the innervessel wall 104. In a further embodiment, the symmetric exhaust 112 mayinclude a number of individual exhaust lines that are arrangedsymmetrically around the inner vessel wall 104. Additionally shown inFIG. 9B is dynamic gas lock (DGL) supply 110 that introduces gas intothe vessel from a location that is proximate to the IF region 157.

FIG. 9C is a simplified schematic view of an embodiment for an EUVvessel 100 having a showerhead 101 with a plurality of nozzles 120 thatare supplied by a gas supply 103. The gas supply 103 may be controlledby a common gas delivery system (not shown). In the embodiment show, theplurality of nozzles 120 are shown to enable a plurality of gas flows140, which introduce gas into the vessel 100 in a direction that is awayfrom the inner vessel wall 104. Also shown in FIG. 9C is a laserradiation pulse 105 that enters a material target region 123, as well asa center supply 106, a perimeter supply 108, and a DGL supply 110.

In a further embodiment shown in FIG. 9D, the showerhead 101 is shown toinclude a number of separately supplied pluralities of nozzles 120 a-120n. The first plurality of nozzles 120 a is shown to be supplied by afirst supply 103 a and is shown to introduce gas into the vessel 100 asa first plurality of gas flows 140 a. The first plurality of nozzles 120is understood to extend along a perimeter of the inner vessel wall 104,according to some embodiments.

The second plurality of nozzles 120 b and the third plurality of nozzles120 c are shown to be configured to be along a similar lateral distanceon the inner vessel wall 104 relative to the IF region 157. However, thesecond and third pluralities of nozzles 120 b and 120 c are shown tohave different perimeter and/or azimuthal positions along the innervessel wall 104. Furthermore, the second plurality of nozzles 120 b isshown to be supplied by a second supply 103 b, whereas the thirdplurality of nozzles 120 c is shown to be supplied by a third supply 103c. As a result, the second and third plurality of nozzles 120 b and 120c may enable control over flow geometries of gas within the vessel 100.

According to some embodiments, each of the pluralities of nozzles 120a-120 n may be supplied by separately controlled gas delivery systems(not shown). As a result, precise control over flow geometries withinthe vessel 100 may be obtained via individual control over thepluralities of nozzles 120 a-120 n. The second, third, and nth pluralityof nozzles, 120 b, 120 c, and 120 n, are shown to enable a second, thirdand nth plurality of flows, 140 b, 140 c, and 140 n, respectively. Eachof the flows 140 a-140 n are shown to be generally in a direction thatis away from the inner vessel 140 through which the flows 140 a-140 nare introduced through.

By way of example and limitation, a flow of gas in the range of 50-500slm may be used for delivery to the supply 103 to enable protection ofinner vessel wall 104. In embodiments having more than one supply, forexample the embodiment shown in FIG. 9D, a flow of the range may bedistributed among the supplies 103 a-103 n. In one embodiment, thesupply 103 may be supplied with about 200 slm of gas. Of course, otherranges of mass flow rates for introducing gas into the vessel 100 may beused to fit the needs of various embodiments, and the example given isnot meant to be limiting.

As previously discussed, protection of inner vessel wall 104 or otherexposed surfaces of the showerhead 101 involves providing flowgeometries of gas within the vessel to suppress Sn vapor flux, SnH₄vapor flux, and other Sn debris flux onto the one or more inner vesselwalls 104. As an example, as a result of the flow geometries produced byeach of the center supply 106, the perimeter supply 108, and theshowerhead 101, there may be a several hundred-fold reduction of SnVapor and SnH₄ debris onto the inner vessel walls 104, according to someembodiments. In other embodiments, for example, further reduction of Sndebris flux may be achieved by using asymmetric vanes/liner structurethat will not have pumping in the area above the EUV collector 102.

According to some embodiments, a range of 10-1,000 nozzles of about 1-10mm in diameter may be used. Depending on the number and sizing ofnozzles, each of the nozzles may be spaced approximately 1-10 cm apart,according to certain embodiments. A gas supply (not shown) andshowerhead body or manifold or plurality of gas lines (not shown) thatprovide uniform and stable mass flux through the plurality of nozzlesmay be used with the embodiments described. Of course, embodiments withmore or less nozzles of differing cross-sectional widths (e.g.,diameters) and spacing may be used without departing from the scope andspirit of the disclosure.

Additionally, according to some embodiments, the plurality of nozzles120 may interface the inner vessel wall 104 such that openings of thenozzles 120 are flush with a plasma-facing surface of the inner vesselwall 104. In other embodiments, the nozzles 120 may protrude into thevessel 100 from the inner vessel wall 104 for example, for a fewmillimetres (not shown). Nozzles 120 that protrude into the inner vesselwall 104 or liner may offer a degree of protection from clogging as aresult of Sn debris during source operation, or during periodic liquidrun-off.

To ensure that a nozzle outflow area does not become contaminated,certain embodiments may include nozzle tips with ceramic (e.g. ZrN)material. In these and other embodiments, hydrogen radicals may besupplied proximate to the nozzle openings and/or tips for cleaning. Inaddition, these embodiments may use EUV induced self-cleaning. In theseand other embodiments, the nozzles 120 may also include a hollow plug ofdifferent material that is placed in a larger opening of the innervessel wall 104.

To prevent or reduce Sn debris from entering the plurality of nozzles120, certain embodiments may include nozzles 120 that are made of aporous medium. As a result, the outflow of gas may be less sensitive toparticles that become deposited on the nozzle opening (not shown). Inother embodiments, a mesh grid that is placed near the nozzle openingcould be included with each nozzle to prevent Sn debris from enteringinto the nozzles 120. In certain embodiments, the mesh grid could beheated for local generation of hydrogen radicals. In yet otherembodiments, nozzles 120 may include a capped showerhead hole from whichgas is introduced sideways along a perimeter of the inner vessel wall104. In these embodiments, a leak flow on the top of the cap may beimplemented to avoid deposition of Sn debris on the cap itself.

FIG. 10 is a simplified schematic view of an embodiment for an EUVvessel 100 having a first showerhead 101 a, a second showerhead 101 band an asymmetric exhaust 132. The inner vessel wall 104 is shown tohave a ceiling region 104 a that is defined as the portion of the innervessel wall 104 (or portion of the first showerhead 101 a that definesportions of the inner vessel wall 104) that lies gravitationally abovethe EUV collector 102. The ceiling region 104 a is shown to include aboundary 134 that defines portions of the inner vessel wall 104 that liegravitationally above the EUV collector 102 and portions that do not. Itshould be appreciated that a shape of boundary 134 will depend upon theshape of the inner vessel wall 104. The boundary 134 is meant to beexemplary and not limiting. For example, boundary 134 may be locatedmore proximal to the IF region 157 or to the EUV collector 102,depending on a shape of the inner vessel wall 104, and an orientation ofthe EUV vessel 100 while the EUV source is being operated.

The asymmetric exhaust 132 is shown to be oriented along the innervessel wall 104 at a location that is generally opposite of the ceilingregion 104 a to promote flow geometries of gas within the vessel 104that are away from the EUV collector 102 and away from the ceilingregion 104 a. In certain embodiments, the asymmetric exhaust 132 may beconfigured to be in a downward leaning direction. Of course, manydifferent orientations for the asymmetric exhaust may be implementedwithout departing from the spirit and scope of the embodiments. Theconfiguration of the asymmetric exhaust 132 shown in FIG. 10 is intendedto be exemplary and not limiting.

For example, the direction in the asymmetric exhaust 132 may be in anupward leaning angle while still being able to promote flow geometrieswithin the vessel 100 that are away from the ceiling region 104 a andaway from the EUV collector 102. Furthermore, in certain otherembodiments, the asymmetric exhaust 132 may be configured to be more orless proximal to the EUV collector 120 than in the embodiment shown. Inaddition, to avoid spitting inside the inner vessel wall 104, a scrubberthat is located remotely downstream towards a pump may be included incertain embodiments (not shown).

The showerhead 101 a is shown to include a first plurality of nozzles120 a that may be separately supplied with gas than a second pluralityof nozzles 120 b of showerhead 101 b. For example, the first pluralityof nozzles 120 a is shown to be supplied by a first supply 103 a,whereas the second plurality of nozzles 102 b is shown to be supplied bya second supply 103 b. The first plurality of nozzles 120 a is alsoshown to extend along a larger lateral length of the inner vessel wall104 than the second plurality of nozzles 120 b. However, in otherembodiments, the opposite may be true. Moreover, the first plurality ofnozzles 120 a is shown to provide a first plurality of flows 140 a thatis greater in number than the second plurality of flows 140 b providedby the second plurality of nozzles 120 b. Again, in other embodiments,the opposite may be true. In certain other embodiments, the first andsecond plurality of nozzles 120 a and 120 b may be supplied by a commongas supply.

Although not shown, the first plurality of nozzles 120 a and the secondplurality of nozzles 120 b may extend circumferentially orperimetrically along the inner vessel wall 104 a certain distance. Incertain embodiments, the first plurality of nozzles 120 a may extendfarther along the perimeter than the second plurality of nozzles 120 b,whereas in other embodiments, the second plurality of nozzles 120 b mayextend farther along the perimeter than the first plurality of nozzles120 a. In other embodiments, the first and second pluralities of nozzles120 a and 120 b may extend a similar distance along the perimeter, ormay extend more or less than the other along given perimeters dependingon the lateral position of the first or second plurality of nozzles 120a and 120 b.

In certain embodiments, the first and second plurality of nozzles 120 aand 120 b may be supplied by a common gas delivery system. In stillother embodiments, each of the nozzles of the first and second pluralityof nozzles 120 a and 120 b may be individually supplied with gas andcontrolled to enable precise control over flow geometries within thevessel 100.

Again, it should be understood that although the inner vessel wall 104of FIG. 10 is shown to include a generally conical shape, there are anynumber of shapes that the inner vessel wall 104 (or the showerhead 101that defines at least a portion of the inner vessel wall 104) mayinclude that do not depart from the scope and spirit of the embodiments.For example, certain embodiments may have inner vessel walls withelliptical, rectangular, or polygonal cross sections. Moreover, theseand other embodiments may include a combination of different types ofsurfaces (e.g., smooth or vane-defined surfaces) that correspond toinner vessel wall shapes that have the aforementioned types of crosssections. Further still, the inner vessel wall may include smoothsurfaces, or surfaces defined by vanes, or have a combination of both.As a result, any number of inner vessel wall 104 shapes may beimplemented in accordance with embodiments having an asymmetric exhaust132 configuration and one or more showerheads 101.

FIG. 11 is a simplified schematic view of an embodiment of an EUV vessel100 that is oriented at angle 119 for operation. Laser radiation pulse105 from a radiation source is shown to enter a material target region123 from which EUV radiation 115 is produced. Virtual laser radiationpath 105 a is shown in the Figure is shown to be a path the laserradiation pulse 105 might take relative to a direction of gravity. As anon-delimiting example, certain embodiments may have the EUV vessel 100oriented such that virtual laser radiation path 105 a has an angle 119of between about 45° and about 80°, depending on the designedconstruction. Nevertheless, it will be understood that the angle 119 mayvary anywhere between 0° and 90°, depending on the particulars of eachapplication.

Also shown in FIG. 11 an inner vessel wall 104 that is connected to anEUV collector 102. The inner vessel wall 104 is shown to extend from theEUV collector 102 to an IF region 157, an upper region 153 that includesportions of the inner vessel wall 104 proximal to the EUV collector 102,as well as a medial region 155, which includes portions of the innervessel wall disposed in between the IF region 157 and the upper region153. An outer vessel wall 121 is shown to surround the inner vessel wall104.

FIG. 12A is a cross-sectional view of an embodiment for an EUV vessel100 that shows a plurality of flow paths for gas being introduced intothe vessel 100 from different supplies and being exhausted by asymmetric exhaust 112 that extends around the perimeter of the innervessel wall 104. In the embodiment shown, the EUV vessel 100 is shown toinclude a center supply 106, a perimeter supply 108, and a DGL supply110. The center supply 106 introduces gas that follows center supplyflow paths 114, which are shown to travel around a material targetregion (not shown) toward symmetric exhaust 112.

Also shown in FIG. 12A is a perimeter supply 108 that introduces gasadjacent a perimeter of the EUV collector 102 that follows perimetersupply flow paths 116. The perimeter supply flow paths 116 are showngenerally to travel inward toward an axis or center of the vessel 100before traveling toward the symmetric exhaust 112. Certain perimetersupply flow paths 116 a are shown to take more circuitous routes withinthe vessel 100, and for example, increase contact instances with theinner vessel wall 104. In particular, perimeter supply flow path 116 ais shown to travel back towards perimeter supply 108 along a path thatis proximate to the ceiling region 104 a of the inner vessel wall 104,which may increase contact of Sn debris with the inner vessel wall 104.

The third gas supply shown in FIG. 12A is a DGL supply 110 thatintroduces gas into the vessel from a region near an IF region 157. Gasintroduced by the DGL supply 110 follows DGL supply flow paths 118. Asindicated in FIG. 12A, the DGL supply flow paths 118 may follow acircuitous route that includes a looping path along the inner vesselwall 104 laterally for some distance. As a result, the DGL supply flowpaths 118 may result in increased contact instances or flux of Sn debrisonto the inner vessel wall 104. For example, since the gas supplies(e.g., center supply 106, perimeter supply 108, and DGL supply 110)shown in FIG. 12A introduce gas that may act as diffusion barriers andcarrier mediums for by-products produced by the laser produced plasma,each representative flow path may at any given time include Sn debris.As a result, while the EUV collector 102 is shown to be protected fromSn debris to a certain extent due to the center supply 106 and perimetersupply 108 that have flow paths 114 and 116, respectively, which are ina general direction that is away from the EUV collector 102, the samemay not be as true for all regions of the inner vessel wall 104.

FIG. 12B is a cross-sectional view of an embodiment for an EUV vessel100 showing a plurality of flow paths 114, 116, 118, and 136 for gasbeing introduced into the vessel from various supplies and beingexhausted by an asymmetrical exhaust 132. The flow paths 114, 116, 118,and 136 are based on computational fluid dynamics (CFD) simulationswhile taking into account plasma gas interactions (PGI). The embodimentis shown to be equipped with a showerhead 101 having a plurality ofnozzles 120. While flows that emanate from the plurality of nozzles 120of the showerhead 101 are present in simulation shown in FIG. 12B, theyhave been omitted from view for clarity.

As shown in FIG. 12B, the EUV vessel 100 includes a center supply 106, aperimeter supply 108, a curtain supply 122, and a DGL supply 110. Alsoshown in FIG. 12B is an asymmetric exhaust 132 that is oriented alongthe vessel 100 at a location that is generally opposite of a ceilingregion 104 a of the inner vessel wall 104 that is gravitationally abovethe EUV collector 102. In general, the asymmetric exhaust 132 is shownto exhaust gas that is introduced via the various supplies in adirection that is both away from the EUV collector 102 and the ceilingregion 104 a of the inner vessel wall 104.

For example, each of the center cone supply flow paths 114 and theperimeter supply flow paths 116 are shown to emanate from proximate theEUV collector 102 and subsequently enter the asymmetric exhaust 132.Unlike the embodiment shown in FIG. 12A, the asymmetric exhaust 132 ofthe embodiment shown in FIG. 12B enables flow geometries that do notsubstantially cycle backwards or loop back along the inner vessel wall104. As a result, the perimeter supply flow paths 116 are able to reducecontact instances of Sn debris being carried by the gas with the innervessel wall 104.

Also shown in FIG. 12B are DGL supply flow paths 118 for gas beingintroduced into the vessel 100 via the DGL supply 110. Each of the DGLsupply flow paths 118 enter and exit the vessel 100 with reducedinstances of looping or cycling back. Again, as a result of thereduction of instances of looping or cycling back, contact instances ofthe gas introduced by the DGL supply 110 and the by-product that it maycarry with the inner vessel wall 104 is reduced.

In the embodiment shown in FIG. 12B, a curtain supply 122 is shown toalso introduce gas into the vessel 100 at a location that is proximateto an IF region 157 of the vessel 100. Gas that is introduced via thecurtain supply 122 is shown to follow curtain supply flow paths 136 thatenters into the vessel 100 as a curtain flow in a lateral direction awayfrom the IF region 157. Gas that is introduced via the curtain supply122 is also shown to subsequently exit the vessel 100 via asymmetricexhaust 132. Much like the other flow paths of the embodiment shown inFIG. 12B, each of the curtain supply flow paths 136 enter and exit thevessel in a manner that reduces contact instances with the inner vesselwall 104. In certain embodiments, the curtain supply 122 may consist ofslit nozzles or an array of nozzles introducing gas flow that issubstantially parallel to the inner vessel wall 104. Although not shown,flow paths that result from gas being introduced by the plurality ofnozzles 120 of the showerhead 101 also enter and exit the vessel withoutsubstantially looping or cycling back toward the inner vessel wall 104.

While the representative flow paths of certain supplies are shown, itshould be appreciated that certain supplies may be excluded from theembodiment shown in FIG. 12B while still retaining the overalldirectionality and flow geometries of gas flowing away from the EUVcollector 102 as well as flowing away from the ceiling region 104 a ofthe inner vessel wall 104. For example, if the curtain supply 122 wereto be excluded from the embodiment shown in FIG. 12B, the remainingcenter supply flow path 114, the perimeter supply flow path 116, and theDGL supply flow path 118 would retain similar overall geometries thatare in a direction that is towards the asymmetric exhaust 132 and awayfrom the EUV collector 102 and the inner vessel wall 104. However, it isnoted that a greater reduction in flow path looping or cycling back isachievable when an asymmetric exhaust 132 is practiced with flows of gasemanating from the inner vessel wall 104 such as that provided by theshowerhead 101.

It should be noted that each of the individual flow paths of 114, 116,118, and 136 are representative and should not be interpreted aslimiting the number of inlets used for each of the center supply 106,the perimeter supply 108, the DGL supply 110, or the curtain supply 122.For example, each of the supplies may include any number of inlets forintroducing gas at their respective locations.

FIG. 13 is a cross-sectional view of Sn concentration gradientscalculated based on CFD simulations of an EUV vessel 100 having aplurality of nozzles 120 (e.g., of a showerhead) distributed laterallyalong an inner vessel wall 104 that introduce gas flows 140 into thevessel 100, according to one simulated embodiment. Each of theshowerhead nozzles 120 are shown to introduce gas flows 140 in adirection that is away from the inner vessel wall 104. As a result,there is a first region 142 of the inner vessel space 100 a immediatelyadjacent to the inner vessel wall 104 that has a Sn concentration aboutan order of magnitude less than that of a second region 144 that extendsfurther into the inner vessel space 100 a. Although not shown on thegradient scale, regions that are further away from the inner vessel wall104, such as a third region 146, are shown to have Sn concentrationlevels that are at least an order of magnitude higher than that of thefirst region 142. Thus, in certain embodiments having nozzles 140 thatintroduce gas flows 140 in a direction that is away from the innervessel wall 104, a level of protection from Sn debris may be obtained.

FIG. 14 is a cross-sectional view of an EUV vessel 100 having a curtainflow nozzle assembly 200 to introduce gas into the vessel 100 as curtainflows, according to one simulated embodiment. The EUV vessel 100 isshown to have a curtain flow nozzle assembly 200 that is locatedadjacent to an inner (plasma-facing) wall of the inner vessel wall 104.The curtain flow nozzle assembly 200 is shown to include a first outlet202, a second outlet 204, and a third outlet 206, each for introducinggas into the vessel. The first outlet 202 is shown in the Figure tointroduce gas in a counter-clockwise direction that is along a perimeter(e.g., circumference in this example) of the inner vessel wall 104,while the second outlet 204 is shown in the Figure to introduce gas in aclockwise direction along the same perimeter. The third outlet 206 isshown to introduce gas in a direction that is generally away from theinner vessel wall 104.

Gas that is introduced by the first outlet 202 of the curtain flownozzle assembly 200 is shown to result in a first curtain flow 212 thatextends from the first outlet 202. Gas that is introduced by the secondoutlet 204 is shown to result in a second curtain flow 214 that extendsfrom the second outlet 204. In certain embodiments such as the one shownin FIG. 14 , an asymmetric exhaust 132 may be oriented along a region ofthe vessel 100 that is opposite of a region of the vessel 100 where thecurtain flow nozzle assembly 200 may be located.

According to the embodiment shown in FIG. 14 , regions within the innervessel space 100 a that are adjacent to the inner vessel wall 104, suchas a first region 208, demonstrate Sn concentration gradients that aremany orders of magnitude lower than that of regions of the inner vesselspace 100 a that further away from the inner vessel wall 104 such as asecond region 210. According to certain embodiments, the first andsecond curtain flows 212 and 214 provide flow geometries within theinner vessel space 100 a that protect the inner vessel wall 104 from Sndebris contamination. In these embodiments, the first and second curtainflows 212 and 214 act as diffusion barriers along the respective wallportions that they travel along. As a result of lowered Sn concentrationgradients proximate to the inner vessel wall 104, contact instances ofSn debris with the inner vessel wall 104 are reduced.

In certain embodiments, a third outlet 206 is also included within thecurtain flow nozzle assembly 200 to provide flow geometries of gaswithin the inner vessel space 100 a that is away from the curtain flownozzle assembly 200 itself. As a result, the curtain flow nozzleassembly 200 is protected from Sn debris contamination. In certain otherembodiments, the third outlet 206 may be excluded from the curtain flownozzle assembly 200.

In some embodiments, the EUV vessel 100 may include a plurality ofcurtain flow nozzle assemblies 200 that are arranged laterally along atleast a portion of the inner vessel wall 104. For example, in certainembodiments, the plurality of curtain flow nozzle assemblies 200 may bearranged laterally within a ceiling region of the inner vessel wall thatis located gravitationally above the EUV collector (not shown). In theseembodiments, the plurality of curtain flow nozzle assemblies may bearranged along a line segment that travels from a region near the EUVcollector to an IF region (e.g., a generatrix, if the vessel isconical).

Although the curtain flow nozzle assembly 200 is shown to be locatedwithin an inner vessel space 100 a (e.g., protruding past the innervessel wall 104), other embodiments may have the curtain flow nozzleassembly 200 configured such that openings of the first, second, andthird outlets 202, 204, and 206 are more flush with the inner vesselwall 104. Additionally, although the embodiment shown includes anasymmetric exhaust 132, it should be understood that other embodimentsmay have a curtain flow nozzle assembly 200 that is practiced withoutthe asymmetric exhaust 132. Moreover, while the embodiment shownincludes smooth surfaces as the inner vessel wall 104, it should beunderstood that certain other embodiments may include inner vessel walls104 that include vanes or surfaces defined by vanes. For example, thevanes may be a separate structure that line the inside of the innervessel wall 104 and project into a volume defined by the inner vesselspace 100 a. In these embodiments, the curtain flow nozzle assembly 200may be integrated into the vane structure, or may be separate from thevane structure.

FIG. 15A is a cross-sectional view of an EUV vessel 100 having ashowerhead 101 and an asymmetric exhaust 132 that shows Sn concentrationwithin the inner vessel space, according to one simulated embodiment.The showerhead 101 is shown to be integrated into the inner vessel wall104 such that it shares a portion of the inner vessel wall 104 as partof its structure. The showerhead 101 is shown to include a plurality ofnozzles 120 that extend around a perimeter of the vessel 100. Each ofthe plurality of nozzles 120 is shown to introduce a gas flow 140 thatis in a direction away from the inner vessel wall 104.

The vessel 100 is also shown to include a center supply 106, a perimetersupply 108, a DGL supply 110, and a curtain supply 122. Snconcentrations in regions of the inner vessel space that are adjacent tothe inner vessel wall 104 are shown to be less than that of regionsfurther away from the inner vessel wall 104 and toward a center regionof the inner vessel space 100 a. For example, a first region 218 that isadjacent to the inner vessel wall 104 is shown to have an Snconcentration that is several orders of magnitude lower than that of asecond region 216 that is further away from the inner vessel wall 104.

FIG. 15B is a cross-sectional view of an EUV vessel 100 having ashowerhead 101 and an asymmetric exhaust 132 that shows Sn depositionrates on an inner vessel wall 104, according to one embodiment andaccording to simulations. According to the embodiment shown, regions ofthe inner vessel wall 104 that are protected by a showerhead 101 areshown to have deposition rates that are orders of magnitude lower thanthat of one or more walls included by the asymmetric exhaust 132. Forexample, there is a region 220 of the ceiling region 104 a of the innervessel wall 104 that is gravitationally above the EUV collector 102 thatis shown to have a minimal deposition rate of Sn debris. In contrast,there is a region 222 of the asymmetric exhaust 132 wall not locatedgravitationally above the EUV collector 102 that is shown to have a Sndeposition rate of about 200-1000 nm/hr or higher (the color bar cutsoff at 1000 nm/hr).

Although embodiments are shown that include a showerhead 101 that havenozzles 140 arranged to encompass a perimeter of the inner vessel wall104, it should be noted that there are other embodiments in which ashowerhead 101 having nozzles 120 that do not cover the entire perimeterof the inner vessel wall that may be implemented without departing fromthe scope and spirit of the embodiments. For example, certainembodiments may include a nozzle arrangement that do not cover theentire perimeter of the inner vessel wall 104, and others that may coverthe entire perimeter, but only for a certain lateral distance along theinner vessel wall 104. Furthermore, although a pattern of nozzle 120arrangement is shown for illustrative purposes, it should be understoodthat any number of nozzle 120 arrangements may be used to provideprotection of the inner vessel walls 104. For example, a greater orlesser number of nozzles 120 may be used than what is shown. Further,the pattern in which the nozzles 120 are arranged may vary in terms ofspacing, uniformity, nozzle diameter, etc., in order to meet the needsof various embodiments.

FIG. 16A is a cross-sectional view of an EUV vessel 100 having a curtainflow supply 201 and an asymmetric exhaust 132 that shows Snconcentration within the inner vessel space, according an embodiment.The vessel 100 is shown to include a plurality of curtain flow nozzleassemblies 200 that are arranged laterally along the inner vessel wall104. Each curtain flow nozzle assembly 200 is shown to include a firstoutlet 202 of gas, a second outlet 204 of gas, and a third outlet 206 ofgas. The first outlet 202 of gas is shown to be curtain flow thattravels along a perimeter of the inner vessel wall 104. The secondoutlet 204 of gas is shown to also be a curtain flow that travels alongthe perimeter of the inner vessel wall 104 counter-directionally to thefirst outlet 202 of gas. The curtain flow nozzle assembly 200 is alsoshown to include a third outlet 206 of gas that introduces gas into thevessel 100 in a direction that is away from the inner vessel wall 104.Additionally, the vessel 100 is shown to include a center supply 106, aperimeter supply 108, a DGL supply 110, and a curtain supply 122.

As a result of the curtain flow supply 201, Sn concentrations in regionsof the inner vessel space 100 a that are adjacent to the inner vesselwall 104 are shown to be less than that of regions further away from theinner vessel wall 104 and toward a center region of the inner vesselspace 100 a. For example, a first region 224 that is adjacent to theinner vessel wall 104 is shown to have a Sn concentration that isseveral orders of magnitude lower than that of a second region 226 thatis further away from the inner vessel wall 104.

FIG. 16B is a cross-sectional view of an EUV vessel 100 having a curtainflow supply and an asymmetric exhaust 132 that shows Sn deposition rateson an inner vessel wall 104, according to one embodiment and accordingto simulations. According to the embodiment shown, regions of the innervessel wall 104 that are protected by a curtain flow supply 201 areshown to have deposition rates that are orders of magnitude lower thanthat of one or more walls included by the asymmetric exhaust 132. Forexample, there is a region 228 of the ceiling region 104 a of the innervessel wall 104 that is gravitationally above the EUV collector 102 thatis shown to have a minimal deposition rate of Sn debris. In contrast,there is a region 230 of the asymmetric exhaust 132 wall not locatedgravitationally above the EUV collector 102 that is shown to have a Sndeposition rate that is several orders of magnitude higher than that ofthe region 228.

Although certain embodiments of EUV vessel that include a showerhead ofa conical shape have been illustrated, it should be understood thatthere are many ways to implement a showerhead with an EUV vessel thatfall within the scope and spirit of the embodiments. For example,certain embodiments may have a showerhead having a separate body ormanifold or a plurality of gas lines that supplies each of the pluralityof nozzles. The body or manifold or plurality of gas lines supplying theshowerhead may be located behind an inner vessel wall (e.g., on thenon-plasma facing side of the inner vessel wall). In certain otherembodiments, the body or manifold of the showerhead may be located onthe plasma-facing side such that the body or manifold of the showerheadmay itself define at least a portion of the inner vessel wall that isexposed to plasma and Sn debris. In still other embodiments, the body ormanifold may be one with the inner vessel wall such that the innervessel wall makes up a portion of showerhead. In these and otherembodiments, the inner vessel wall may include embedded paths orchannels that include the body or manifold or plurality of gas linesthat supply the showerhead. Furthermore, in these embodiments, theshowerhead may be integrated into the inner vessel wall. In certainother embodiments, the showerhead may have a distributed body ormanifold or may be without a body or manifold. As mentioned previously,for example, the plurality of nozzles of the showerhead may be suppliedby gas lines that interface with each of the nozzles. In this manner,the showerhead may simply be defined as the plurality of nozzlesconfigured in a showerhead-like manner.

In one embodiment having a generally conical shape, an EUV sourceincludes a vessel having an upper cone region, a focal cone region, anda medial cone region disposed between the upper cone region and thefocal cone region, wherein the upper cone region and the focal coneregion are disposed at opposite ends of the vessel. The embodimentincludes an EUV collector having a reflective surface that is disposedinside the vessel where the reflective surface is directionallyconfigured to face the focal cone region of the vessel. The embodimentalso includes a conical showerhead that is disposed along at least aportion of the inner vessel wall. The conical showerhead includes aplurality of nozzles that introduce gas into the vessel. A plurality ofexhausts oriented proximate to the focal cone region for removing gasintroduced into the vessel is also included in the embodiment such thatgas introduced into the vessel is caused to flow away from the EUVcollector.

In a further embodiment having a generally conical shape, an EUV sourceincludes a vessel having an upper cone region, a focal cone region, anda medial cone region disposed between the upper cone region and thefocal cone region, wherein the upper cone region and the focal coneregion are disposed at opposite ends of the vessel. The embodimentincludes an EUV collector having a reflective surface that is disposedinside the vessel with the reflective surface being directionallyconfigured to face the focal cone region of the vessel. The embodimentincludes a first gas source disposed proximate to the reflective surfaceof the EUV collector having a plurality of inlets for introducing gasinto the vessel. The embodiment also includes a conical showerheaddisposed along at least a portion of the inner vessel wall having aplurality of nozzles for introducing gas into the vessel. An exhaustthat is oriented at an asymmetric position between the upper cone regionand the focal cone region is also included by the embodiment forexhausting gas from the vessel. In certain embodiments, the asymmetricexhaust may be oriented at a downward leaning angle, for example, towarda direction of gravity. In these and other embodiments, the asymmetricexhaust may be oriented such that it opposes a region proximate to aceiling area of the inner vessel wall that is gravitationally above theEUV collector.

In a further embodiment having a generally conical shape, an EUV sourceincludes a vessel having an upper cone region, a focal cone region, anda medial cone region disposed between the upper cone region and thefocal cone region, wherein the upper cone region and the focal coneregion are disposed at opposite ends of the vessel. The embodimentincludes an EUV collector having a reflective surface that is disposedinside the vessel with the reflective surface being directionallyconfigured to face the focal cone region of the vessel. The embodimentincludes a first gas source disposed proximate to the reflective surfaceof the EUV collector having a plurality of inlets for introducing gasinto the vessel. A second gas source disposed laterally at leastpartially along a portion of the inner vessel wall having a plurality ofnozzle assemblies is also included in the embodiment. According to thisembodiment, each of the nozzle assemblies include a first outlet and asecond outlet for introducing gas into the vessel with the first outletconfigured to introduce gas in a first direction that is away from asecond direction in which the second outlet is configured forintroducing gas.

FIG. 17 depicts a further embodiment of a radiation source, which may beprovided in the form of an EUV source SO. The EUV source SO comprises achamber, which may be provided in the form of an EUV vessel 100. The EUVvessel 100 comprises an inner wall 104 and a material target plasmaregion 111. The EUV source SO comprises a radiation collector, which maybe provided in the form of EUV collector 102, arranged in the EUV vessel100. The EUV collector 102 is configured to collect radiation e.g. EUVradiation 115, emitted at the material target region 111 and to directthe collected EUV radiation 115 to an intermediate focus (IF) region157. A focal point 157 a of the EUV collector (which may be referred toas intermediate focus 157 a) is located at or near the intermediatefocus region 157, as described herein. The EUV source SO, e.g. the EUVvessel 100, comprises a debris mitigation system. The debris mitigationsystem may comprise or be provided in the form of the center supply 106,the perimeter supply 108, and/or the DGL supply 110, as described above.The debris mitigation system, e.g. the DGL supply, is configured todirect a first gas flow from the intermediate focus region 157 towardsthe material target region 111. The first gas flow may be or comprisegas supplied by the DGL supply 110. It will be appreciated that the term“first gas flow” and “gas supplied by the DGL supply” may beinterchangeably used.

The debris mitigation system is configured to direct a second gas flowfrom a portion of the inner vessel wall 104 into the EUV vessel 100. Forexample, the debris mitigation system comprises the showerhead 101,which includes a plurality of nozzles 120 for introducing the second gasflow or gas into the EUV vessel 100. The second gas flow may be orcomprise gas supplied by the showerhead 101. It will be appreciated thatthe term “second gas flow” and “gas supplied by the showerhead” may beinterchangeably used.

In the example depicted in FIG. 17 , the showerhead 101 is provided inthe form of the first showerhead 101 a and the second showerhead 101 b,as described above. The first showerhead 101 a comprises the firstplurality of nozzles 120 a and the second showerhead 101 b comprises thesecond plurality of nozzles 120 b. The first and second plurality ofnozzles 120 a, 120 b may be provided for directing the gas supplied bythe showerhead 101, e.g. the first and second showerheads 101 a, 101 b,into the EUV vessel 100. It will be appreciated that in otherembodiments the second flow of gas may be supplied by the curtain flowsupply, as described above.

The EUV vessel 100 comprises a guiding device, which may be provided inthe form of a flow splitter 150. The flow splitter 150 is arranged inthe EUV vessel 100 such that the gas supplied by the DGL supply 110 isdirected around the flow splitter 150. The EUV vessel 100 comprises anexhaust 132 for removing gas supplied by the debris mitigation systemfrom the EUV vessel. The exhaust 132 may be configured for removingdebris that is carried by the gas from the EUV vessel 100. The exhaust132 is arranged to extend from a portion of the inner wall 104 of theEUV vessel 100, for example at an azimuthally asymmetric position, asdescribed above. The EUV vessel 100 depicted in FIG. 17 is similar tothose depicted in FIGS. 10, 12B, 15A and 15B and may comprise any of thefeatures described above in relation to FIGS. 10, 12B, 15A and 15B.

FIG. 18A depicts simulated flow paths of the gas supplied by the DGLsupply 110, the showerhead 101, the center supply 106 and/or theperimeter supply 108. As described above, the center supply 106introduces gas that follows center supply flow paths 114. The perimetersupply 108 introduces gas that follows perimeter supply flow paths 116(depicted together with the center supply flow paths 114 in FIG. 18A).Gas introduced by the DGL supply 110 follows DGL supply flow paths 118.Each of the showerhead nozzles 120 (e.g. each of the first and secondplurality of nozzles 120 a, 120 b) introduces gas that follows flowpaths 140, as indicated in FIG. 18A.

The debris mitigation system may be configured to direct a third gasflow or gas from a position at or proximate to the flow splitter 150 (orthe intermediate focus region 157) in the EUV vessel to the materialtarget region 111. The debris mitigation system may be configured todirect the third gas flow from a position on the internal wall 104 tothe material target region 111. The debris mitigation system maycomprise the curtain supply 122, as described above. Gas that isintroduced via the curtain supply 122 follows curtain supply flow paths136. It will be appreciated that the term “third gas flow” and “gassupplied by the curtain supply” may be interchangeably used.

The gas supplied by the center supply 106 and/or the perimeter supply108 may be or be comprised in a fourth gas flow. It will be appreciatedthat the term “fourth gas flow” and “gas supplied by the center supplyand/or perimeter supply” may be interchangeably used. The debrismitigation system, e.g. the center supply 106, the perimeter supply 108,may be configured to direct the fourth flow of gas from the EUVcollector 102 towards the target material region 111. The flow splitter150 may be configured to reduce or prevent the interaction between thegas supplied by the DGL supply 110 and the gas supplied by the centersupply 106 and/or the perimeter supply 108. The flow splitter 150 may beconfigured to prevent formation of a jet of the gas supplied by the DGLsupply 110, e.g. towards EUV collector 102.

A flow rate of the gas supplied by the DGL supply 110 may be selected toprevent debris from entering the intermediate focus region 157. A flowrate of the gas supplied by the DGL supply 110 may be selected dependingon a gas supplied by the DGL supply 110, a density or pressure of thegas supplied by the DGL supply 110, a size of debris, e.g. particulatedebris, or a velocity of debris and/or a direction of debris diffusionin the EUV vessel of the radiation source SO. Additionally oralternatively, the flow rate of the gas supplied by the DGL supply 110may be selected depending on the arrangement or geometry of the DGLsupply. For example, the flow rate of the gas supplied by the DGL supply110 may be selected dependent on a number of the openings of the DGLsupply 110, a cross-sectional width (e.g., diameter) of each opening ofthe DGL supply 110 and/or a cross-sectional width (e.g., diameter),periphery or dimension of the intermediate focus region 157. Forexample, a maximum velocity of the gas supplied by the DGL supply 110may be in the range of about 1000 to 3000 m/s.

The gas supplied by the DGL supply 110 may have a flow rate in the rangeof about 5 to 30 slm. Debris may include particulate debris, such as forexample Sn clusters, Sn microparticles, Sn nanoparticles, and/or Sndeposits, molecular and/or atomic debris, such as for example Sn vapor,SnH_(x) vapor, Sn atoms, Sn ions, as described above. A flow rate ofabout 7 slm may be sufficient to prevent molecular and/or atomic debrisgenerated in the EUV vessel 100 from entering the intermediate focusregion 157. To suppress particulate debris from reaching theintermediate focus region 157, flow rates of the gas supplied by the DGLsupply 110 larger than 7 slm may be required. For example, to suppressparticulate debris from reaching the intermediate focus region 157, aflow rate of larger than 15 slm of the gas supplied by the DGL supply110 may be required. At a flow rate of larger than 15 slm, such as forexample 20 slm, an interaction between the gas supplied by theshowerhead 101 and/or the curtain supply 122 may be observed. Thisinteraction may lead to debris in the EUV vessel 100 to be spread beforeit is removed from the EUV vessel 100, with some of the gas, by theexhaust 132 and/or may result in contamination of the internal wall 104of the EUV vessel 100.

The flow splitter 150 is configured to reduce interaction between thegas supplied by the DGL supplied 110 and the gas supplied by theshowerhead 101 and/or between the gas supplied by the DGL supply 110 andthe gas supplied by the curtain supply 122. By reducing the interactionbetween the gas supplied by the DGL supply 110 and the gas supplied bythe showerhead 101 and/or between the gas supplied by the DGL supply 110and the gas supplied by the curtain supply 122, spreading of the debrisbefore removal from the EUV vessel by the exhaust 132 may be reduced.This may further reduce contamination in the EUV vessel, e.g. theinternal wall of the EUV vessel. The debris mitigation system, e.g. theshowerhead 101 and/or the curtain supply 122, may be configured orarranged such that the gas supplied by showerhead 101 and/or the curtainsupply 122 directs the debris towards the exhaust 132.

FIG. 18B depicts a simulated debris concentration in the EUV vessel 100.It can be seen from FIG. 18B that the provision of the flow splitter150, the showerhead 101 and/or the curtain supply 122 allows the debristo be directed towards the exhaust 132, while reducing contamination inthe EUV vessel 100, e.g. the inner wall 104 of the EUV vessel.

Referring to FIG. 17 , the flow splitter 150 is arranged such that thegas supplied by the DGL supply 110 is symmetrically directed around theflow splitter 150. The flow splitter 150 may be configured to diffuse orspread, e.g. symmetrically diffuse or spread, the gas supplied by theDGL supply 110. By arranging the flow splitter 150 in the EUV vessel100, recirculation of at least some of the gas supplied by the DGLsupply 110, for example due to the interaction of the gas supplied bythe DGL supply 110 with gas supplied by the showerhead 101, curtainsupply 122, center supply 106 and/or perimeter supply 108, in the EUVvessel 100 may be reduced. This may lead to less debris being depositedon the inner wall 104 of the of the radiation source SO. Additionally oralternatively, by arranging the flow splitter 150 in the EUV vessel 100such that the gas supplied by the DGL supply 110 is directed around theflow splitter 150, contamination of the flow splitter 150, e.g. withdebris, may be reduced or prevented.

The flow splitter 150 may be arranged in the EUV vessel 100 of theradiation source SO to maintain the maximum velocity of the gas suppliedby the DGL supply 110 at a first location in the radiation source SO. Atthe first location the velocity of the gas supplied by the DGL supply110 may correspond (or substantially correspond) to a maximum velocityof the gas supplied by the DGL supply 110, for example when no flowsplitter is arranged in the EUV vessel 100 of the radiation source SO.The flow splitter 150 may be arranged in the EUV vessel 100 of theradiation source SO to diffuse or spread the gas supplied by the DGLsupply 110 to prevent or reduce recirculation of at least some gassupplied by the DGL supply 110, for example in a direction towards theintermediate focus 157 a. The flow splitter 150 may be arranged in theradiation source SO to diffuse or spread the gas supplied by the DGLsupply 110 at a second location, which may be spaced or remote from theintermediate focus point 157 a. The flow splitter 150 may be arranged inthe EUV vessel 100 of the radiation source SO so that the maximumvelocity of the gas supplied by the DGL supply 110 is reduced at thesecond location and/or a minimum velocity of gas supplied by the DGLsupply 110 that may be directed in a direction away from theintermediate focus 157 a is increased.

Referring to FIG. 17 , the flow splitter 150 is arranged in the EUVvessel 100 to extend across a portion of the EUV vessel 100. Forexample, the flow splitter 150 may be arranged to extend at leastpartially along an optical axis OA of the EUV collector 102. In otherwords, the flow splitter 150 may be arranged in the EUV vessel 100 suchthat a central or longitudinal axis A of the flow splitter 150 (depictedin FIG. 19A) coincides with at least a part of the optical axis OA ofthe EUV collector 102. The EUV vessel 100 may comprise a conical shape,which extends from the intermediate focus region 157 towards or near theEUV collector 102. The conical shape of the EUV vessel 100 may allow asymmetrical arrangement of the flow splitter 150 in the EUV vessel 100.It will be understood that the exemplary EUV vessel described herein isnot limited to comprising a conical portion. For example, the EUV vesselor a portion thereof may have any suitable shape, for example, to reducethe volume of the EUV vessel, without obstructing the EUV radiation 115.

The flow splitter 150 is arranged at or in proximity of the intermediatefocus region 157. For example, the flow splitter 150 is arranged at orin proximity of the intermediate focus region 157 to enable the flowsplitter 150 to act on the gas supplied by the DGL supply 110.

The flow splitter 150 may be arranged at a distance from theintermediate focus point 157 a. The distance of the flow splitter 150from the intermediate focus point 157 a may be in the region of 5 to 15cm. However, it should be understood that the arrangement of the flowsplitter 150 in the radiation source SO is not limited to such adistance and other values for the distance may be selected. For example,the distance may be selected dependent on space available at or inproximity of the intermediate focus region and/or thermal loads that mayact on the flow splitter 150, e.g. due to the radiation at theintermediate focus region. In other words, the distance may be selectedsuch that any thermal effects on the flow splitter 150, such as forexample melting of the flow splitter 150, are minimized or prevented.

The flow splitter 150 may be arranged to extend at least partially alongthe central or longitudinal axis of the EUV vessel 100, which in thisexample corresponds to at least a part of the optical axis OA of the EUVcollector 102. This arrangement may allow the flow splitter 150 tosymmetrically direct the gas supplied by the DGL supply 110 around theflow splitter 150, for example, to reduce or prevent the interactionbetween the gas supplied by the DGL supply 110 and the gas supplied bythe showerhead 101 and/or between the gas supplied by the DGL supply 110and the gas supplied by the curtain supply 122. Additionally, thisarrangement may allow the flow splitter 150 to symmetrically direct thegas supplied by the DGL supply 110 around the flow splitter 150, forexample, to reduce or prevent the interaction between the gas suppliedby the DGL supply 110 and the gas supplied by the center supply 106and/or the perimeter supply 108 and/or may prevent the formation of ajet of the gas supplied by the DGL supply 110, e.g. towards EUVcollector 102.

The exemplary flow splitter 150 depicted in FIG. 19A is arranged totaper from a first 150 a end towards a second end 150 b. The first end150 a of the flow splitter 150 may comprise or define an enlargedportion. The flow splitter 150 may be arranged in the EUV vessel 100such that the first end 150 a, e.g. the enlarged portion, of the flowsplitter 150 is positioned distal from the intermediate focus region157. The second end 150 b of the flow splitter 150 may define orcomprises a pointed portion 150 a. The flow splitter 150 may be arrangedin the EUV vessel 100 such that the second end 150 b, e.g. the pointedportion, of the flow splitter is positioned at or proximal to theintermediate focus region 157. The exemplary flow splitter 150 depictedin FIG. 19A comprises a conical shape.

FIG. 19B depicts a further exemplary arrangement of the flow splitter150. The flow splitter 150 depicted in FIG. 19B is similar to thatdepicted in FIG. 19A. The first end 150 a of the flow splitter 150defines or comprises the enlarged portion. The second end 150 b of theflow splitter 150 comprises or defines a rounded portion. The exemplaryflow splitter depicted in FIG. 19B may be considered as comprising asubstantially truncated conical shape. It should be understood that theflow splitter disclosed herein is not limited to a conical or truncatedconical shape. In other examples, the flow splitter may comprise aconical or truncated conical shape having one or more flat portions.Alternatively, the flow splitter may comprise a spiral or helical shape.

Referring to FIGS. 19A and 19B, an extension or dimension of the flowsplitter 150, for example along the longitudinal or central axis A ofthe flow splitter 150, may be selected depending on a dimension, volumeand/or shape of the EUV vessel 100. The extension or dimension of theflow splitter 150 may be selected such that the flow splitter 150interacts with the gas supplied by the DGL supply 110 and/or the flowsplitter directs the gas supplied by the DGL supply 110 around the flowsplitter 150, as described above, e.g. when the flow splitter 150 isarranged in the EUV vessel 100. An exemplary extension or dimension ofthe flow splitter 150 along the longitudinal or central axis A of theflow splitter 150 may comprise about 3 to 30 cm, e.g. 10 to 20 cm.However, it should be understood that the exemplary flow splitterdisclosed herein is not limited to such an extension or dimension.

The EUV source SO may include a heating element 152, which may be partor comprised in the flow splitter 150. The heating element 152 may beconfigured to increase a temperature of the flow splitter 150, forexample to increase an amount of the gas supplied by the DGL supply 110that is directed around the flow splitter 150.

The heating element 152 may be configured to increase the temperature ofthe flow splitter 150 to or above a first temperature at which anincreased amount of the gas supplied by the DGL supply 110 is directedaround the flow splitter 150. For example, an increase of thetemperature of the flow splitter 150 to or above the first temperaturemay result in an increase of the velocity of at least some of the atomsof the gas supplied by the DGL supply 110, e.g. when at least a portionof the gas supplied by the DGL supply 110 comes into contact with theflow splitter 150. An increase of the temperature of the flow splitter150 to or above the first temperature may cause heat to be transferredto a portion of the gas supplied by the DGL supply 110 that comes intocontact with the flow splitter 150. The transfer of heat to the portionof the gas supplied by the DGL supply may cause the gas of the portionto expand and/or a viscosity of the gas of the portion to increase. Inother words, the gas of the portion of the gas supplied by the DGLsupply 110 that comes into contact with the flow splitter 150 maycomprise an increased viscosity. The gas of the portion of gas suppliedby the DGL supply 110 comprising the increased viscosity may act onanother portion of the gas supplied by the DGL supply 110, which isincident on the flow splitter 150 and/or cause the other portion of thegas supplied by the DGL supply 110 to be directed around the flowsplitter 150. In other words, due to the increased viscosity of the gasof the portion of the gas supplied by the DGL supply 110, the effectivedimension of the flow splitter 150 may be considered as being increasedrelative to the actual dimension of the flow splitter 150.

The first temperature may be equal to or larger than the meltingtemperature of the fuel used to create the plasma 107. In other words,the first temperature may be selected dependent on the fuel used tocreate the plasma 107. For example, when tin is used as a fuel, theheating element 152 may be configured to increase the temperature of theflow splitter 150 to temperatures of about or larger than 230° C. (whichlargely corresponds to the melting temperature of tin). For temperaturesbelow 200° C., any fuel, e.g. tin, deposited on the flow splitter 150may be solid. The solid fuel may cause diffraction or block at least aportion of the EUV radiation 115 directed towards the intermediate focus157 a.

The heating element 152 may be configured to maintain the temperature ofthe flow splitter 150 below a second temperature. At or above the secondtemperature, diffusion of debris that may be present on the flowsplitter occurs or increases. For example, at the second temperature orabove the second temperature diffusion of debris that may be present onthe flow splitter 150 may be increased. For example, the diffusioncoefficient of tin vapor in a hydrogen atmosphere may increase withincreasing temperature. By maintaining the temperature of the flowsplitter 150 below the second temperature, diffusion of debris in theEUV vessel 100 may be reduced. The amount of debris on the flow splitter150 may be considered to be small, for example, due to the flow splitter150 being arranged in the EUV vessel 100 to direct the gas supplied bythe DGL supply 110 around the flow splitter 150.

The heating element 152 may be embedded in the flow splitter 150. Itwill be appreciated that in other embodiments, the heating element maybe provided separately. In such embodiments, the heating element may bearranged to increase the temperature of the flow splitter. The heatingelement 152 may be provided in the form of a resistive heating element.It will be appreciated that in other embodiments, the flow splitter maybe inductively heated and/or the heating element may be provided in theform of an electromagnetic element, e.g. a coil or the like. Anelectronic oscillator, e.g. a radio frequency generator, may be providedto generate electric currents in the electromagnetic element, which mayresult in heat being generated in the electromagnetic element.

Referring to FIGS. 17 and 19C, in some embodiments, the flow splitter150 may be configured for cooling by a coolant. The flow splitter 150may be cooled, for example to reduce the thermal loads that may act onthe flow splitter 150, e.g. due to the EUV radiation at the intermediatefocus region 157. The flow splitter 150 may be cooled to maintain atemperature of the flow splitter 150 below a melting temperature of thefuel used to create the plasma 107. This may preventdistribution/diffusion of liquid fuel that may be present on the flowsplitter 150 onto the internal wall 104 or any other component of theradiation source SO. As described above, the amount of debris that maybe present on the flow splitter 150 is considered to be small, forexample, due to the flow splitter 150 being arranged in the EUV vessel100 to direct the gas supplied by the DGL supply 110 around the flowsplitter 150.

The coolant may be supplied by a coolant source 154. For example, theflow splitter 150 may comprise a channel 156 for receiving the coolantfrom the coolant source 154 and/or flowing the coolant through the flowsplitter 150. The flow splitter 150 may be configured for connection tothe coolant source 154. The coolant source 154 may be configured tosupply the flow splitter 150 with a coolant. For example, the coolantsource 154 may be configured to supply the flow splitter 150 with acoolant to decrease a temperature of the flow splitter 150, e.g. below amelting temperature of the fuel fused to create the plasma 107 and/orthe second temperature, as described above. The coolant may be providedin the form of a coolant fluid, e.g. a coolant liquid or a coolant/coldgas etc. It will be appreciated that the flow splitter may be configuredfor being cooled by the coolant instead to or addition to comprising theheating element 152.

FIG. 20 schematically depicts a further embodiment of the EUV source SO.The EUV source SO depicted in FIG. 20 is similar to that depicted inFIG. 17 . The exemplary flow splitter 150 of the radiation source SOdepicted in FIG. 20 includes a plurality of further openings 158, whichmay be provided in the form of nozzles or slits. The plurality offurther openings 158 (or each further opening of the plurality offurther openings 158) may be configured to direct a fifth gas flow 160towards the EUV collector 102. The fifth gas flow may comprise a flowrate in the range of about 1 to 50 slm. The plurality of furtheropenings 158 may be arranged on the flow splitter 150 such that thefifth gas flow 160 from the plurality of further openings 158 interactswith the gas supplied by the DGL supply 110. The interaction between thegas supplied by the DGL supply 110 and the fifth gas flow 160 may director push the gas supplied by the DGL supply 110 into proximity with theinner wall 104 of the EUV vessel 100. The provision of the plurality offurther openings 158 for directing the fifth gas flow 160 towards theEUV collector 102 may lead to an increased spreading of the gas suppliedby the DGL supply 110. The increased spreading of the gas supplied bythe DGL supply 110 may result in a reduced or suppressed interactionbetween the gas supplied by the DGL supply 110 and the gas supplied bythe showerhead 101 and/or between the gas supplied by the DGL supply 110and the gas supplied by the curtain supply 122.

The plurality of further openings 158 may be circumferentially,peripherally and/or axially arranged on the flow splitter 150. In otherwords, the plurality of further openings 158 may be arranged to extendaround the flow splitter 150 and/or in a direction of the central orlongitudinal axis A of the flow splitter 150. The plurality of furtheropenings 158 may be symmetrically arranged on the flow splitter 150, forexample to cause a symmetric flow of the gas supplied by the DGL supply110 and/or the fifth gas flow 160 around the flow splitter 150.

The DGL supply 110 may be configured to supply the fifth gas flow 160 tothe flow splitter 150. For example, the flow splitter 150 may beconnected or connectable to the DGL supply 110 e.g. to enable supply ofthe fifth gas flow 160 to the flow splitter 150. It will be appreciatedthat in other example, the debris mitigation system may comprise afurther gas supply system, which may be configured to supply the gasflow to the flow splitter. The flow splitter may be connected orconnectable to the further gas supply system, e.g. to enable supply ofthe gas flow to the flow splitter. Although the flow splitter 150depicted in FIG. 20 comprises a plurality of further openings 158, itwill be appreciated that in other embodiments the flow splitter maycomprise a single further opening, which may be configured to direct thefifth gas flow towards the EUV collector.

FIG. 21 schematically depicts a further embodiment of the EUV source SO.The EUV source SO depicted in FIG. 21 is similar to that depicted inFIG. 17 . The exemplary EUV source SO depicted in FIG. 21 comprises adebris receiving surface 162 a, which may be part of or provided by abar or obscuration bar 162. The bar 162 may be arranged in the EUVvessel 100 to reduce or prevent debris from reaching the intermediatefocus region 157. The bar 162 may be arranged to intersect or extendacross the optical axis OA of the EUV collector 102. In thisarrangement, the bar 162 can be considered to obscure the direct line ofsight of debris, which may include ballistic particulate debris, and/orof a portion of the laser radiation 105, e.g. the portion of the laserradiation 105 that passes through the material target region 111. Inother words, the bar 162 may be configured to reflect the portion of thelaser radiation 105 away from the intermediate focus region 157 of theEUV source SO.

In the exemplary EUV source depicted in FIG. 21 , the flow splitter 150is arranged between the bar 162 and the intermediate focus region 157.In this arrangement, the bar 162 is arranged to extend over or overlapwith at least a portion or all of the flow splitter 150. For example,the bar 162 may be arranged to extend over or overlap with the enlargedportion of the first end 150 b of the flow splitter 150 so that debrisgenerated by the plasma 107 is incident on the debris receiving surface162 a of the bar 162. In other words, flow splitter 150 may be arrangedin the shadow of the bar 162.

Although in the exemplary EUV source SO depicted in FIG. 21 , the debrisreceiving surface 162 a was described as being part of the bar 162, itwill be appreciated that in other embodiments of the EUV source, such asfor example any of those described in relation to FIGS. 17 and 20 , thedebris receiving surface 162 a may be provided by or be part of the flowsplitter 150. In such case, the flow splitter 150 may comprise any ofthe features of the bar 162, described above. Additionally, the flowsplitter 150 may be configured such that the flow splitter 150 is ableto withstand the heat or heat/thermal load created by the plasma 107 orthat of the EUV radiation 115 at the intermediate focus region 157. Theflow splitter 150 may be configured to reflect the portion of the laserradiation 105 that passes through the material target region 111, awayfrom the intermediate focus region 157. For example, when the debrisreceiving surface 162 a is provided by the flow splitter 150, theextension or dimension of the flow splitter 150, e.g. in a directionperpendicular and/or parallel to the central or longitudinal axis A ofthe flow splitter 150, may be increased relative to the extension ordimension, e.g. in a direction perpendicular and/or parallel to thecentral or longitudinal axis A, of a flow splitter 150 that is used incombination with the bar 162.

The first, second, third, fourth and/or fifth gas flow may comprisehydrogen gas. It will be appreciated that in other embodiments anothergas or a mixture of gases may be used. For example, in other embodimentsthe first, second, third, fourth and/or fifth gas flow may compriseargon or helium gas.

The material of the flow splitter 150 may be selected to be corrosionresistant, e.g. to be resistant against corrosion by the fuel in theenvironment in the EUV source SO, e.g. the hydrogen environment in theEUV source SO. The material of the flow splitter 150 may be selected tobe resistant to the thermal loads acting on the flow splitter, e.g. dueto the EUV radiation 115 in the radiation source SO and/or the plasma107, and/or to the increase of the temperature of the flow splitter 150to or above the first temperature, as described above. The exemplaryflow splitter 150 may comprise a metal or metal alloy. For example, thematerial of flow splitter may be or comprise molybdenum, tungsten,aluminium, stainless steel, copper or an alloy thereof. The flowsplitter 150 may comprise a metal or metal alloy surface. The metal ormetal alloy surface of the flow splitter may lead to an improvedrecombination of hydrogen radials, which may be present in the radiationsource SO. For example, hydrogen (H₂) molecules may split into hydrogenradicals due to their absorption of heat and/or radiation or ioncollisions. The hydrogen radicals may be beneficial for removing debris,e.g. tin, from the internal wall 104 of the radiation source. Thepresence of hydrogen radicals may cause contamination in the EUV vessel100, such as spitting of fuel, for example, when the hydrogen radicalsdiffuse into layers of fuel in the EUV vessel 100 that are liquid. Byproviding the flow splitter with a metal or metal alloy surface, therecombination of hydrogen radicals may be improved and/or thecontamination, e.g. spitting of fuel, in the EUV vessel reduced.

It will be appreciated that in other embodiments the flow splitter maycomprise another material, such as for example a ceramic material. Theceramic material may comprise a silicon dioxide, zirconium nitride, orzirconium oxide material. Although specific reference may be made inthis text to embodiments in the context of a lithographic apparatus,embodiments of the invention may be used in one or more otherapparatuses. Embodiments may form part of a mask inspection apparatus, ametrology apparatus, or any apparatus that measures or processes anobject such as a wafer (or other substrate) or mask (or other patterningdevice). These apparatus may be generally referred to as lithographictools. Such a lithographic tool may use vacuum conditions or ambient(non-vacuum) conditions.

The term “at least part of the lithographic apparatus” may be consideredas encompassing the illumination system IL, patterning device MA, and/orthe projection system PS. The term “radiation source” may be consideredas encompassing the laser 162. The term “prevent” may be considered toencompass substantially prevent. The term “intermediate focus region”may be considered to encompass a region at and/or near the intermediatefocus point. The term “EUV radiation” may be considered to encompasselectromagnetic radiation having a wavelength within the range of 4-20nm, for example within the range of 13-14 nm. EUV radiation may have awavelength of less than 10 nm, for example within the range of 4-10 nmsuch as 6.7 nm or 6.8 nm.

Although FIG. 1 depicts the radiation source SO as a laser producedplasma LPP source, any suitable source may be used to generate EUVradiation. For example, EUV emitting plasma may be produced by using anelectrical discharge to convert fuel (e.g. tin) to a plasma state. Aradiation source of this type may be referred to as a discharge producedplasma (DPP) source. The electrical discharge may be generated by apower supply which may form part of the radiation source or may be aseparate entity that is connected via an electrical connection to theEUV radiation source SO.

Although the flow splitter 150 was described as being arranged in a EUVvessel comprising the asymmetric exhaust 132, it will be appreciatedthat in other embodiments the flow splitter may be used in an EUV vesselcomprising a symmetric exhaust, such as for example depicted in FIGS. 9Bto 9C, 12A. Additionally or alternatively, the flow splitter may be usedin an EUV vessel comprising a curtain flow supply, such as for exampledepicted in FIGS. 16A and 16B. The flow splitter may also be used in theEUV vessel depicted in FIGS. 9A and 11 .

Additionally, it will be understood that each of the foregoingembodiments may be practiced with a temperature control system thatconditions the inner vessel walls 104, as well as other componentsincluded in the EUV vessel 100 to achieve certain temperatures. Certainportions of the inner vessel walls 104, for example, may be kept attemperatures that are below a melting point of Sn, whereas otherportions may be kept at temperatures that are above the melting point ofSn. In these and other embodiments, the temperatures of each of theregions of the inner vessel wall 104 may also be changed, or cyclebetween temperatures that are above and below the melting point of Sn.

In an embodiment, there is provided a radiation source comprising: achamber comprising a plasma formation region; a radiation collectorarranged in the chamber, the radiation collector configured to collectradiation emitted at the plasma formation region and to direct thecollected radiation to an intermediate focus region; a debris mitigationsystem configured to direct a first gas flow from the intermediate focusregion towards the plasma formation region; and a guiding devicearranged in the chamber such that the first gas flow is directed aroundthe guiding device.

In an embodiment, the guiding device is arranged such that the first gasflow is symmetrically directed around and/or diffused by the guidingdevice. In an embodiment, the debris mitigation system is configured todirect a second gas flow from the radiation collector towards the plasmageneration region. In an embodiment, the guiding device is configured toreduce interaction between the first gas flow and the second gas flow.In an embodiment, the guiding device is configured to preventinteraction between the first gas flow and the second gas flow. In anembodiment, the guiding device is configured to prevent formation of ajet of the first gas flow towards the radiation collector. In anembodiment, the guiding device is arranged in the chamber to extend atleast partially along an optical axis of the radiation collector. In anembodiment, the guiding device is arranged at or in proximity of theintermediate focus region. In an embodiment, the guiding device isarranged to taper from a first end of the guiding device towards asecond end of the guiding device, the first end comprising an enlargedportion and the second end comprising a pointed portion or roundedportion. In an embodiment, the guiding device is arranged in the chambersuch that the first end of the guiding device is positioned distal fromthe intermediate focus region and the second end of the guiding deviceis positioned at or proximal to the intermediate focus region. In anembodiment, the guiding device comprises a plurality of openings, theplurality of openings configured to direct a third gas flow towards theradiation collector. In an embodiment, the plurality of openings arearranged on the guiding device such that the third gas flow from theplurality of openings interacts with the first gas flow to direct orpush the first flow of gas into proximity with at least a portion of thechamber. In an embodiment, the guiding device comprises a heatingelement configured to increase a temperature of the guiding device. Inan embodiment, the heating element is configured to increase thetemperature of the guiding device to a first temperature at which anincreased amount of the first gas flow is directed around the guidingdevice and/or to maintain the temperature of the guiding device below asecond temperature at or above which diffusion of debris that is presenton the guiding device increases. In an embodiment, the guiding device isconfigured for cooling by a coolant, the coolant being supplied by acoolant source. In an embodiment, the radiation source comprises adebris receiving surface, the debris receiving surface being arranged inthe chamber to reduce or prevent debris from reaching the intermediatefocus region. In an embodiment, the debris receiving surface is arrangedto intersect or extend across the optical axis of the radiationcollector. In an embodiment, the guiding device is arranged between thedebris receiving surface and the intermediate focus region. In anembodiment, the debris receiving surface is arranged to extend over oroverlap with at least a portion or all of the guiding device so thatdebris generated at the plasma formation region is incident on thedebris receiving surface. In an embodiment, the debris receiving surfaceis comprised in, part of or provided by the guiding device.

In an embodiment, there is provided a method of reducing debrisdeposition in a radiation source, the method comprising: directing afirst gas flow from an intermediate focus region of the radiation sourcetowards a plasma generation region of the radiation source; anddirecting the first gas flow around a guiding device arranged in achamber of the radiation source.

In an embodiment, there is provided an extreme ultraviolet (EUV) source,comprising: a vessel having an inner vessel wall and an intermediatefocus (IF) region; an EUV collector disposed inside the vessel connectedto the inner vessel wall, the EUV collector including a reflectivesurface, the reflective surface configured to directionally face the IFregion of the vessel; a showerhead disposed along at least a portion ofthe inner vessel wall, the showerhead including a plurality of nozzlesconfigured to introduce gas into the vessel, the showerhead having atleast one inlet configured to supply the gas into the showerhead; andone or more exhausts configured to remove gas introduced into thevessel, the one or more exhausts being oriented along at least a portionof the inner vessel wall so that the gas is caused to flow away from theEUV collector.

In an embodiment, the EUV source further comprises a material targetregion disposed within the vessel for generating plasma radiation, theplasma radiation being collected by the reflective surface of the EUVcollector and directed toward the IF region for entrance into at leastpart of a lithographic apparatus, wherein introduction of the gas intothe vessel via the plurality of nozzles enables protection of the innervessel wall from deposition of material. In an embodiment, the pluralityof nozzles is oriented along at least a portion of an inner surface ofthe inner vessel wall in a direction that faces away from the innersurface of the inner vessel wall. In an embodiment, the inner vesselwall has a conical, cylindrical, or polyhedral shape. In an embodiment,the showerhead extends perimetrically and laterally along at least aportion of the inner vessel wall. In an embodiment, the EUV sourcefurther comprises an outer vessel wall surrounding the vessel, the outervessel wall including one or more exhaust vents. In an embodiment, theshowerhead includes one or more zones, each of the one or more zonesincluding at least a portion of the plurality of nozzles, each of theone or more zones being separately supplied with gas to enableseparately controllable zones for introducing gas into the vessel. In anembodiment, the inner vessel wall is defined by smooth surfaces, vanesurfaces, or a combination of smooth surfaces and vane surfaces.

In an embodiment, there is provided an extreme ultraviolet (EUV) source,comprising: a vessel having an inner vessel wall and an intermediatefocus (IF) region; an EUV collector disposed inside the vessel connectedto the inner vessel wall, the EUV collector including a reflectivesurface that is configured to directionally face the IF region of thevessel; a first gas source configured to introduce gas into the vessel,the first gas source including a first plurality of inlets, the firstplurality of inlets disposed proximate to the reflective surface of theEUV collector; a showerhead disposed along at least a portion of theinner vessel wall, the showerhead including a plurality of nozzlesconfigured to introduce gas into the vessel, the showerhead having atleast one inlet configured to supply gas into the showerhead; and anexhaust disposed along the inner vessel wall at an azimuthallyasymmetric position and configured to exhaust gas from the vessel.

In an embodiment, the exhaust is further oriented proximate to a firstregion of the inner vessel wall, the first region of the inner vesselwall generally opposing a second region of the inner vessel wall that islocated gravitationally above the EUV collector, the exhaust enablinggas introduced by the first gas source and the plurality of nozzles toflow away from the second region while the EUV source is operational. Inan embodiment, the plurality of nozzles is distributed at leastpartially along a region of the inner vessel wall that is locatedgravitationally above the EUV collector. In an embodiment, the pluralityof nozzles is oriented along an inner surface of the inner vessel wallin a direction that is away from the inner surface of the inner vesselwall, the orientation of the plurality of nozzles enabling a flow of gasthat is at least partially directed away from at least a portion of theinner surface of the inner vessel wall. In an embodiment, the pluralityof nozzles are disposed at least partially along a ceiling region of theinner vessel wall that is located gravitationally above the EUVcollector, the plurality of nozzles oriented in a direction that facesaway from the ceiling region, the introduction of the gas by theplurality of nozzles providing a diffusion barrier adjacent the ceilingregion for excluding debris. In an embodiment, the showerhead includesone or more zones, each of the one or more zones including at least aportion of the plurality of nozzles, each of the one or more zones beingseparately supplied with gas to enable separately controllable zones forintroducing gas into the vessel. In an embodiment, the inner vessel wallhas a conical, cylindrical, or polyhedral shape.

In an embodiment, there is provided an extreme ultraviolet (EUV) source,comprising: a vessel having an inner vessel wall and an intermediatefocus (IF) region; an EUV collector disposed inside the vessel connectedto the inner vessel wall, the EUV collector including a reflectivesurface, the reflective surface configured to directionally face theintermediate focus region of the vessel; a vessel wall gas sourcedisposed laterally at least partially along the inner vessel wall, thevessel wall gas source including a plurality of nozzle assemblies, eachof the plurality of nozzle assemblies having at least a first outlet anda second outlet configured to introduce gas into the vessel, the firstoutlet configured to introduce gas in a first direction that is awayfrom a second direction in which the second outlet is configured tointroduce gas; and an exhaust configured to exhaust gas introduced intothe vessel, the exhaust being proximate to the IF region to enable gasintroduced by the vessel wall gas source to flow away from the EUVcollector.

In an embodiment, the first direction and the second direction in whichgas is introduced by the first outlet and the second outlet,respectively, of each the plurality of nozzle assemblies are oriented atleast partially along a perimeter of the inner vessel wall to enablecurtain flows of gas along the perimeter of the inner vessel wall. In anembodiment, at least a portion of the plurality of nozzle assembliesfurther includes a third outlet configured to introduce gas into thevessel, the third outlet configured to introduce gas away from the innervessel wall. In an embodiment, the plurality of nozzle assemblies isdistributed at least partially along a first region of the inner vesselwall that is located gravitationally above the EUV collector while theEUV source is operational, and wherein the exhaust is further orientedproximate to a second region of the inner vessel wall that opposes thefirst region of the inner vessel wall to enable gas that is introducedinto the vessel to flow away from the first region of the inner vesselwall. In an embodiment, the inner vessel wall has a conical,cylindrical, or polyhedral shape.

In an embodiment, there is provided a radiation source comprising: achamber comprising an inner wall and a material target region; aradiation collector arranged in the chamber, the radiation collectorconfigured to collect radiation emitted at the material target regionand to direct the collected radiation to an intermediate focus region; adebris mitigation system configured to direct a first gas flow from theintermediate focus region towards the material target region, the debrismitigation system configured to direct a second gas flow from a portionof the inner wall of the chamber into the chamber; a guiding devicearranged in the chamber such that the first gas flow is directed aroundthe guiding device; and an exhaust configured to remove gas supplied bythe debris mitigation system from the chamber.

In an embodiment, the exhaust is arranged to extend from a portion ofthe inner wall of the chamber at an azimuthally asymmetric position. Inan embodiment, the debris mitigation system comprises a showerheadarranged along at least a portion of the inner wall of the chamber, theshowerhead including a plurality of nozzles configured to introduce thesecond gas flow into the chamber. In an embodiment, the guiding deviceis configured to reduce interaction between the first gas flow and thesecond gas flow. In an embodiment, the debris mitigation system isconfigured to direct a third gas flow from a position at or proximate tothe guiding device in the chamber towards the material target region. Inan embodiment, the guiding device is configured to reduce interactionbetween the first gas flow and the third gas flow. In an embodiment, thedebris mitigation system is configured to direct a fourth gas flow fromthe radiation collector towards the material target region. In anembodiment, the guiding device is configured to reduce interactionbetween the first gas flow and the fourth gas flow. In an embodiment,the guiding device is arranged to taper from a first end of the guidingdevice towards a second end of the guiding device, the first endcomprising an enlarged portion and the second end comprising a pointedportion or rounded portion. In an embodiment, the guiding device isarranged in the chamber such that the first end of the guiding device ispositioned distal from the intermediate focus region and the second endof the guiding device is positioned at or proximal to the intermediatefocus region. In an embodiment, the guiding device is arranged in thechamber to extend at least partially along an optical axis of theradiation collector. In an embodiment, the guiding device comprises aplurality of openings, the plurality of openings configured to direct afifth gas flow towards the radiation collector. In an embodiment, theplurality of openings are arranged on the guiding device such that thefifth gas flow from the plurality of openings interacts with the firstgas flow to direct or push the first flow of gas into proximity with atleast a portion of the inner wall of the chamber. In an embodiment, theguiding device comprises a heating element configured to increase atemperature of the guiding device. In an embodiment, the heating elementis configured to increase the temperature of the guiding device to afirst temperature at which an increased amount of the first gas flow isdirected around the guiding device and/or to maintain the temperature ofthe guiding device below a second temperature at which diffusion ofdebris that is present on the guiding device increases. In anembodiment, the guiding device is configured for cooling by a coolant,the coolant being supplied by a coolant source. In an embodiment, theradiation source comprises a debris receiving surface, the debrisreceiving surface arranged in the chamber to reduce or prevent debrisfrom reaching the intermediate focus region. In an embodiment, thedebris receiving surface is comprised in, part of or provided by theguiding device.

In an embodiment, there is provided a radiation system comprising alaser and a radiation source as described herein. In an embodiment,there is provided a lithographic system comprising a lithographicapparatus arranged to project a pattern from a patterning device onto asubstrate, and a radiation system as described herein arranged toprovide at least some of the radiation to the lithographic apparatus.

In an embodiment, there is provided a method of reducing debrisdeposition in a radiation source, the method comprising: directing afirst gas flow from an intermediate focus region of the radiation sourcetowards a material target region of the radiation source; directing asecond gas flow from a portion of an inner wall of a chamber of theradiation source into the chamber; directing the first gas flow around aguiding device arranged in the chamber of the radiation source; andremoving gas from the chamber.

In an embodiment, there is provided a radiation source comprising: achamber comprising an inner wall and a material target region; aradiation collector arranged in the chamber, the radiation collectorconfigured to collect radiation emitted at the material target regionand to direct the radiation beam of the collected radiation to anintermediate focus region; a debris mitigation system comprising a firstgas supply system and a second gas supply system; an exhaust configuredto remove gas supplied by the debris mitigation system from the chamber,wherein the first gas supply system is configured to direct a first gasflow from the intermediate focus region towards the material targetregion or the plasma formation region, the first gas supply systemcomprising one or more openings arranged to direct the first gas flow ina direction substantially opposite to a propagation direction of theradiation beam into the chamber, and wherein the second gas supplysystem comprises one or more openings arranged to direct the second gasflow in a direction substantially perpendicular or tilted under an angleto the propagation direction of the first gas flow.

In an embodiment, the second gas supply system comprises a pair ofcounter gas flow jets. In an embodiment, the first gas flow and thesecond gas flow interact via their momentum exchange such that asubstantially unidirectional gas flow field is established towards theexhaust. In an embodiment, the second gas supply system is arranged inproximity of the intermediate focus region, downstream of the first gasflow. In an embodiment, the first and the second gas supply systems arearranged such that, in close vicinity of an interception point of thefirst and second flows, a velocity of the first gas flow issubstantially equal to the velocity of the second gas flow. In anembodiment, a guiding device is arranged in the chamber such that thefirst gas flow is directed around the guiding device. In an embodiment,the radiation source comprises a showerhead disposed along at least aportion of the inner vessel wall, the showerhead including a pluralityof nozzles configured to introduce gas into the vessel, the showerheadhaving at least one inlet configured to supply the gas into theshowerhead; and one or more exhausts configured to remove gas introducedinto the vessel, the one or more exhausts being oriented along at leasta portion of the inner vessel wall so that the gas is caused to flowaway from the EUV collector. In an embodiment, the exhaust is disposedalong the inner vessel wall at an azimuthally asymmetric position andconfigured to exhaust gas from the vessel.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications. Possible other applications include the manufactureof integrated optical systems, guidance and detection patterns formagnetic domain memories, flat-panel displays, liquid-crystal displays(LCDs), thin-film magnetic heads, etc.

Although specific reference may have been made above to the use ofembodiments in the context of optical lithography, it will beappreciated that embodiments of the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

As will be appreciated, aspects of one or more embodiments herein mayincorporated into one or more other embodiments herein as, for example,a substitution or modification.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The descriptions above are intended to beillustrative, not limiting. Thus it will be apparent to one skilled inthe art that modifications may be made to the invention as describedwithout departing from the scope and equivalents of the claims set outbelow.

The invention claimed is:
 1. A radiation source comprising: a chambercomprising an inner wall and a material target region; a radiationcollector arranged in the chamber, the radiation collector configured tocollect radiation emitted at the material target region and to directthe collected radiation as a beam of the collected radiation to anintermediate focus region; a debris mitigation system comprising: afirst gas supply system configured to direct a first gas flow from theintermediate focus region towards the material target region or a plasmaformation region, the first gas supply system comprising one or moreopenings arranged to direct the first gas flow in a directionsubstantially opposite to a propagation direction of the radiation beam,and a second gas supply system comprising one or more openings arrangedto direct a second gas flow in a direction substantially perpendicularor tilted under an angle to the propagation direction of the first gasflow such that the first and second gas flows interact and such that thesecond gas flow confines at least part of the first gas flow to a regionbetween the one or more openings and the intermediate focus region; anexhaust configured to remove gas supplied by the debris mitigationsystem from the chamber, wherein the one or more openings arranged todirect the second gas flow are located between the intermediate focusregion and the exhaust and wherein an opening of the exhaust is locatedbetween the intermediate focus region and the material target or plasmaformation region.
 2. The radiation source according to claim 1, whereinthe second gas supply system comprises a pair of counter gas flow jets.3. The radiation source according to claim 1, wherein the first gas flowand the second gas flow interact via their momentum exchange such that asubstantially unidirectional gas flow field is established towards theexhaust.
 4. The radiation source according to claim 1, wherein anopening of the one or more openings of the second gas supply system isarranged in proximity of the intermediate focus region and is locatedbetween the intermediate focus region and the material target region. 5.The radiation source according to claim 1, wherein the first and thesecond gas supply systems are arranged such that, in close vicinity ofan interception point of the first and second gas flows, a speed of thefirst gas flow is substantially equal to the speed of the second gasflow.
 6. The radiation source according to claim 1, further comprising aflow splitter in the chamber, the flow splitter arranged such that thefirst gas flow is directed around the flow splitter.
 7. The radiationsource according to claim 1, wherein the exhaust is disposed along theinner wall at an azimuthally asymmetric position and configured toexhaust gas from the chamber.
 8. The radiation source according to claim1, wherein the radiation is extreme ultraviolet (EUV) radiation.
 9. Aradiation system comprising: a laser; and the radiation source accordingto claim
 1. 10. A lithographic system comprising a lithographicapparatus arranged to project a pattern from a patterning device onto asubstrate, and the radiation source according to claim 1 arranged toprovide at least some of the radiation to the lithographic apparatus.11. A radiation source comprising: a chamber comprising an inner walland a material target region; a radiation collector arranged in thechamber, the radiation collector configured to collect radiation emittedat the material target region and to direct the collected radiation as abeam of the collected radiation to an intermediate focus region; adebris mitigation system comprising: a first gas supply systemconfigured to direct a first gas flow from the intermediate focus regiontowards the material target region or a plasma formation region, thefirst gas supply system comprising one or more openings arranged todirect the first gas flow in a direction substantially opposite to apropagation direction of the radiation beam, and a second gas supplysystem comprising one or more openings arranged to direct a second gasflow in a direction substantially perpendicular or tilted under an angleto the propagation direction of the first gas flow such that the firstand second gas flows interact; an exhaust configured to remove gassupplied by the debris mitigation system from the chamber, wherein theone or more openings arranged to direct the second gas flow are locatedbetween the intermediate focus region and the exhaust and wherein anopening of the exhaust is located between the intermediate focus regionand the material target or plasma formation region; and a showerheaddisposed along at least a portion of the inner wall, the showerheadincluding a plurality of nozzles configured to introduce gas into thechamber, the showerhead having at least one inlet configured to supplythe gas into the showerhead, wherein at least one nozzle is locatedbetween the opening of the exhaust and the material target or plasmaformation region.
 12. A method comprising: collecting, using acollector, radiation emitted at a material target region in a chamber ofa radiation source and directing the collected radiation as a beam ofthe collected radiation to an intermediate focus region in the radiationsource; directing a first gas flow from the intermediate focus regiontowards the material target region or a plasma formation region in thechamber of the radiation source, using one or more openings arranged todirect the first gas flow in a direction substantially opposite to apropagation direction of the radiation beam in the chamber; directing asecond gas flow in a direction substantially perpendicular or tiltedunder an angle to a propagation direction of the first gas flow usingone or more openings such that the first and second gas flows interactand the second gas flow confines at least part of the first gas flow toa region between the one or more openings and the intermediate focusregion; and removing from the chamber, via an exhaust, gas supplied intothe chamber by the first and/or second gas flows, wherein the one ormore openings arranged to direct the second gas flow are located betweenthe intermediate focus region and the exhaust and wherein an opening ofthe exhaust is located between the intermediate focus region and thematerial target or plasma formation region.
 13. The method according toclaim 12, wherein the second gas flow involves a pair of counter gasflow jets.
 14. The method according to claim 12, wherein the first gasflow and the second gas flow interact via their momentum exchange suchthat a substantially unidirectional gas flow field is establishedtowards the exhaust.
 15. The method according to claim 12, wherein anopening of the one or more openings of the second gas supply system isarranged in proximity of the intermediate focus region and is locatedbetween the intermediate focus region and the material target region.16. The method according to claim 12, wherein, in close vicinity of aninterception point of the first and second gas flows, a speed of thefirst gas flow is substantially equal to the speed of the second gasflow.
 17. The method according to claim 12, wherein the first gas flowis directed around a flow splitter in the chamber.
 18. The methodaccording to claim 12, comprising: introducing gas into the chamberusing a showerhead disposed along at least a portion of an inner wall ofthe chamber, the showerhead including a plurality of nozzles configuredto provide the gas into the chamber, wherein the showerhead has at leastone inlet configured to supply the gas into the showerhead; and removinggas introduced into the chamber using one or more outlets, the one ormore outlets oriented along at least a portion of the inner wall so thatthe gas is caused to flow away from the collector.
 19. The methodaccording to claim 12, wherein the exhaust is disposed along an innerwall of the chamber at an azimuthally asymmetric position and configuredto exhaust gas from the chamber.
 20. A device manufacturing methodcomprising: providing at least some radiation using the method of claim12 to a lithographic apparatus; and projecting, in the lithographicapparatus, a pattern from a patterning device onto a substrate.